Hi mwillems. two questions if you wouldnt mind.
1) when you say "The JRSS mixing won't be correct, but it might allow you to do some manual mixing in PEQ", would there be a difference in sound
quality between a 4 channel set up like the one I have, and a 'true' 2.1 setup (via the 6 channel output)? FYI, I am currently using PEQ for my bass
management.
JRSS mixing for 4-channel output assumes that you want the four channels to be L, R, SL, and SR and mixes accordingly (4 channel output is 4.0, not 3.1). Meaning that it a) doesn't perform bass management and b) will mix LFE channels from 5.1 or 7.1 material into all 4 channels. If you're doing your bass management by hand, you should probably disable JRSS mixing and do the rest of your mixing by hand too. If you don't mix the surrounds back into the mains you might lose info.
This is where use case is an example of how a "stereo in a 4-channel container" option would be very useful, because then you could use JRSS within the constraints of your device. You'd still need some manual mixing, but less of it.
2) So when you say logical channels, it does not matter whether the manufacturer wired it to output the same audio signal or not? If yes, can I just opt
for an interface with 4 or 6 "physical outputs". I am just being gadget-greedy
No, that's not what I mean. By "logical channels" I meant the number of channels that device's ASIO driver reports as being available to software. For many devices that's the number of discrete analog outputs + the number of discrete digital outputs + (maybe) a loopback/DAW channeI or two (depending on the device or manufacturer). But there's no guarantee that it works that way for any particular device. Unfortunately, the only way to be sure that your device can support 6-channel output is to buy a device that a) has six discrete analog outputs or b) is known to aggregate analog and digital channels as described (most Steinberg devices do that, so do some M-audio devices, but it looks like your specific focusrite unit does not).
f the manufacturer hard-wired 2 outputs to mirror each other, that won't get you any additional channels, because in that case it's very likely that they aren't separately addressable in software. So if your device has "six" output channels but outputs 5 and 6 are driven off of a splitter from channels 3 and 4, then it's likely that the ASIO driver would only report 4-channels, which would cause any software attempting to open it for 6-channel output to error out. I had an M-audio card like that. It had "four" analog outputs and two digital outputs, but two of the analog outputs were mirrors of the input and were not addressable. Because it aggregated analog and digital channels, though, it presented as a four-channel device rather than a two channel device.