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Oh I'm sorry, I read over your post quickly and did not properly understand what you were trying to do.
It's a 2 channel (stereo) system which is split over 8 channels.
 
The "2 channels inside a 7.1 container" should only send audio to the front left/right channels, which doesn't help with your setup.
 
 
I think you will have to use an 8 channel output and use the parametric EQ to downmix everything, with JRSS disabled.
I'll look things up for a proper downmix and see if I can come up with a solution.
 
It's a bit difficult for me to test though, as I only have a 2 channel setup.

It would be helpful to know exactly what "2 channels (inside 7.1 channel container)" does when playing a multi-channel (5.1, etc.) recording. I have wasted a lot of time trying to figure it out.

"2 channels inside a 7.1 container" downmixes to stereo, and then adds six blank channels.  It shows "converting 6 channels to 8 channels" with a 5.1 source because the output of the downmix is eight channels, it's just that the last six are blank.  

I have a bi-amped setup with a stereo sub, but I like to control my sub routing so I use "2 channel in a 5.1 container" with JRSS mixing.  It downmixes all audio to stereo, and gives me four blank channels. Then I do my channel routing/sub-mixing in parametric equalizer.  "2 channel in 7.1 container" with JRSS mixing should do exactly what you want: everything downmixed to stereo with six blank channels so you can do your channel routing and crossovers in PEQ.  The center channel does occasionally sound a little soft in my setup, but is normally just fine.

If the center channel sounds very soft that probably means that the center channel isn't getting downmixed correctly (i.e. if JRSS mixing is off that will happen, or if something is glitched with the setting).  I can confirm that "2 channel in a 5.1 container" works exactly as expected, and all downmixing is handled correctly.  I just did a quick test with 2 channel in a 7.1 container with JRSS mixing on, and it seemed to work correctly in my setup, but I haven't "lived with it" like you have.

EDIT: Or if I am understanding what mwillems has written, you would simply be able to use "2 channels inside a 7.1 container" with JRSS enabled and copy the channels, without everything else in the Parametric EQ I listed. (the downmix)

That's exactly what I do.  I use 2 channel in a 5.1 container, and copy my left and right onto the blank channels (S, C, SL, SR) and then do my crossovers, etc.

Well that certainly simplifies things, without having to calculate what the correct downmix from 7.1 to 2.0 is...

It's the point of the x channels in the x container options.  It allows you to specify the up or downmixing target while still having blank channels for custom mixing.  I kind of wish that the mixing target and output container were able to be specified separately, as there are currently some unachievable combinations that could be useful (i.e. 2.1 in a 4-channel container would be useful for some setups, or 4 channel in a 7.1 container, etc.).

Here is a partial listing of the filtering that I have configured. Left and Right initially contain the full stereo signal, that is, with a true stereo source.

L/R = left and right woofers
Center/Sub = Left and right lower midranges
RL/RR = Left and right upper midranges
SL/SR = Left and right tweeter

1) With the PEQ off there are just 3 output lines (RMS, left and right). That confirms that the remaining 6 channels have no data (good).

Update from previous post: when I added the center channel to the L and R channels I must have had only the 7.1 channel option selected (no down-mixing). Sorry to mislead the investigation.

Conclusion: the 2 channels (inside 7.1 channel container) option is blanking channels 3 through 8 but not mixing properly (see A below).

2) Yes, I do toggle playback when making setting changes (other than PEQ filter changes). For overkill I frequently shutdown MC and restart.

3) Please review the attached output format information.

New data:

A) I set the output format to "2 channels (inside 5.1 channel container)". In my configuration the filtered tweeter output is missing (channels seven and eight) but there is enough information in the remaining output to figure out what is happening. In this configuration the center channel is present, however, it floats around like I described earlier. Based on this test I think that the mixing for the 7.1 option is incomplete.

This raises a new question. Why does the mixing produce sound that floats? If this is normal, expected behavior then I may be trying to implement a configuration that won't be acceptable. I never experienced anything like that when playing 5.1 material through my CD/DVD player using only L/R stereo output.

I moved my trouble shooting to another computer to help eliminate variables. This system has a conventional 2 speaker audio system, that is, no PEQ filtering is used. The PC motherboard outputs to a stereo receiver. I have the receiver connected to only the L/R (green) output.

With 2/7.1 + JRSS mixing selected the center channel (voice) data is very low.

With 2/5.1 + JRSS mixing selected the center channel data is present at normal volume.

Clearly 2/7.1 is not mixing correctly.

More importantly, with 2/5.1 + JRSS mixing selected the center channel (voice) data does not float. That implies that the mixing is not working within the PEQ system properly.

I'll check in the coming week whats up with the 2 in 7.1 option, its probably an easy fix somewhere.

Glad we've started to get to the bottom of it.  It looks like the 2 in 7.1 wasn't working correctly just with 5.1 sources, which would explain why it seemed to work for me when I tested it yesterday (I was using a 7.1 source).  

The 2/5.1 option does not work correctly with the PEQ. With the PEQ enabled with multiple filters the center channel data floats.

That hasn't been my experience with that setting.  I use quite a few PEQ filters (obviously I have a bi-amp filter bank not that different from yours), and haven't experienced the center channel "floating" like you're describing.  The voice data does move around a little, but it's typically things that are in the original surround mix (i.e. it's always correlated to what's happening on the screen, voices will sound like they're coming from the left when the person speaking is at the left edge of the screen, etc.). That's achieved in the original mix by moving some of the dialog out of the center channel and into one of the mains; that kind of thing is why you heard some vocal content when the center was entirely missing (not all voice is in the center channel).  But I'm guessing that's not what you mean when you say the center info is "floating around."

I can think of two avenues for inquiry:

1) It may be that the center channel mixing in 2/5.1 container is glitched, and I haven't noticed it in my own setup over the last year.  That's entirely possible: my speakers are only about 6 feet apart and my listening position about 12 feet away, so I tend to get a slightly "too stable" phantom center.  

You mentioned above that you got the "float" even when you didn't use JRSS, but just mixed the center into the two sides manually using PEQ, right?  An easy way to test would be to change output format to 7.1 with no up or downmixing.  Then, copy the center channel to two stereo channels from your setup (overwriting what was already there).  Then mute all other channels.  You could then just listen to the center channel info by itself in stereo on one "stage" of your setup and see if it still floats.  If I were you, I'd try it with different stages of my setup (i.e. try it with the two vocal range drivers first, but then with some of the other driver stereo pairs as well).  Whether it floats or not under those circumstances, it will help isolate what is happening when; if it floats with nothing but the center channel playing you may want to investigate what I mention in the next paragraph.

2) When you run your setup with conventional stereo music does the "stereo illusion" typically work from your listening position (i.e. does most of the sound seem to be coming from directly in between the speakers, rather than seeming to be localized at each speaker)?  If not that may be the issue; the Haas precedence effect makes having a stable phantom center challenging if you're different distances from your speakers, or the timing between stereo channels is off to some extent.  http://en.wikipedia.org/wiki/Precedence_effect.  In that situation even slight variations in the "position" of sound in the mix will cause the center to "collapse" into one side or the other, which causes stereo content that should sound "centered" to "jump" around from left to right to center, etc.  In your case, it might be more noticeable when the mid-high frequency drivers are engaged because HF is much more positional and is a big part of how we "spatialize" sound.  That would also possibly account for why the issue "went away" when you tried different speakers (presumably located in a different room with a different listening position, etc.)

I know I'm probably not telling you anything you don't already know, but I wanted to mention this issue because it could cause the kind of issue you're describing (in case you hadn't already thought about it and ruled it out).

I kind of wish that the mixing target and output container were able to be specified separately, as there are currently some unachievable combinations that could be useful (i.e. 2.1 in a 4-channel container would be useful for some setups, or 4 channel in a 7.1 container, etc.).

+1

With this configuration the sound is perfectly stable and centered. Note that I did not make any changes to the PEQ filtering.

So you mean that you conducted the "copy and mute" experiment as described, but left the rest of your filters in place (just verifying the methodology)?  If that's the case, then to summarize (please correct anything that's incorrect in my restatement):

1) Using JRSS downmixing along with subsequent PEQ filtering gives you an unstable center channel, and
2) Turning off JRSS downmixing, and manually adding center channel information to the mains in PEQ gives you an unstable center channel, but
3) Leaving JRSS downmixing on, but disabling PEQ, gives you a stable center channel (with a different set of speakers), and
4) Turning off JRSS and copying the center channel to a stereo pair in PEQ (overwriting what was already there) and listening to just the center channel in stereo gives you a stable center image.

That seems to suggest that there's something specifically goofy in the channel summing (whether JRSS or PEQ derived), but not all the time (see number 3), and not necessarily in other channel mixing options, like copy (see number 4).  If it weren't for number 2), I'd conclude that the issue was somewhere in the interaction between JRSS and PEQ, but number 2) rules that out.  That is indeed odd, and hopefully will give the devs something to go on if the channel summing is bugged.  

I tried last night to replicate the issue, but my center remains stubbornly centered while using both JRSS and substantial PEQ filtering. and I can't think of any other things to rule out.  

Any other folks who use stereo downmixing and/or bi-ampers (or tri-ampers, or quad-ampers) out there having similar issues with the center?  

Hopefully the devs will have some ideas.

Mixing is completely independent of the PEQ. The output format conversions including resampling and mixing are always performed long before the audio reaches the PEQ. Mixing does not change when you add a PEQ, so something in your PEQ must be causing this.
Since you have a rather long list of filters as it seems, I would suggest to try to isolate it step by step, instead of just enabling/disabling PEQ as a whole.

JRSS downmixing just merges the center into the L/R channels with a -3dB attenuation, there is no other special processing here.
If even manually adding the center to your mains causes the same symptoms, I'm somewhat inclined to blame either your source audio or something in your audio setup. The mixing code is really quite straight forward.

Hmm, if that's the case then I'm stumped. Hopefully dvogel can step through the filters and figure out what might be causing it.

If it's not the channel mixing, and it's not the speaker placement, that only really leaves asymmetrical volume or timing fluctuations (whether due to an unfortunate combination of PEQ filters or something else in the signal path) as a potential cause of an unstable center image.  I'm not sure what could be causing variable asymmetrical volume or timing fluctuations that wouldn't also lead to much more noticeable issues with stereo playback almost immediately.  Maybe a limiter somewhere in the PEQ chain, combined with some boost that isn't offset (or isn't offset until after the limiter)?  Just brainstorming.

Please review attached PEQ

I don't think this is likely to be causing the center channel issue, but just out of curiosity, how do you offset all that boost?

3) and 4) almost lead me to think it is a process update issue because the floating center got considerably worse with the full PEQ active.

Yes, I use internal volume with a limit and I'm working on some modifications to address the gain.

I have tested the 19.0.124 update with a 5.1 video. The center channel is present. That's good. However, the center channel still floats. For example, the voice of a person speaking at the center of the video image sounds like it is properly centered for a second or two and then shifts left or right for a couple of seconds and then back to center, etc. Also, the overall volume seems to be attenuated by several dB.

I think this is the video you're watching rather than anything MC is doing.

Please realize that I'm not going asking unsubstantiated questions.
It's the same for all. You identified a 3 second rotation of some channel (center?) in a previous post although I can't find the post. Has that been fixed with this release?

Perhaps I should back up and ask a simple question. Does this format produce a simple, stable stereo output from a multichannel source? If it contains some remnants of 5.1 surround sound them I'm wasting everyone's time.

The more channels you mix into a stereo signal, the quieter it has to be to prevent clipping. (distortion)

Why is the volume lower than is was previously when it was essentially the same as music playback?

Muting combinations of channels was revealing.

Mute L/R/C/Sub
    Stable center

Mute L/R/RL/RR
     Unstable center

Mute C/Sub
     Stable center

getting to the culprit...

Mute C
     Stable center

I have to set the volume to 100% to achieve what is normally about 1/2 volume.

So even with just one channel muted (Center) the center stabilizes.

That is correct. The "center" is stable when the Center channel is muted.

What happens if you only mute "sub"?

With only the Sub channel muted the "center" is almost stable, that is, some of the Center channel is present on the side that it shouldn't be.

The only time that the "image" is rock solid is when the Center channel is muted.

The Center and Sub channels feed left and right midranges respectively. Please keep in mind that the filtering works correctly when playing 2 channels sources.

For reference (stereo setup):
L/R - woofers
Center/Sub - lower midranges
RL/RR - upper midranges
SL/SR - tweeters

Something I would also like to see, is the option to normalize everything relative to the volume of a 7.1 signal. (reduce stereo playback by 13.5dB, and 5.1 playback by 1.4 dB)

Volume Leveling should make this irrelevant, as it is told about the volume reduction due to mixing and should compensate to create a even volume level, even for downmixes.

Regarding the other issues, I reduced the LFE for an upcoming build to be energy neutral in a stereo downmix, and also reverted the peak level change, as it didn't account for other factors in the mixing, which resulted in driving it into clipping.