Plugins are fun. But moderation is key. Too much can make things a mess. As for headphone crossfeed, my philosophy is that crossfeed is there to compensate for under-performing amps and under-performing headphones. You're playing with the LCD-3 and HD800 so I assume you have a really good amp with nice soundstage. At that point crossfeed is far less beneficial and really not needed or wanted. I like having crossfeed available, but I rarely use it.
Crossfeed or HRTF are not about compensation for bad headphones - they're about the fact that a sound which is hard-panned over to one side will only be heard in that ear.
With speakers, when you pan sounds all the way over to the left or right, you still hear that sound in your other ear as well, but at a lower level and with a slight delay. It's part of how our brain locates where sounds are coming from. Crossfeed fixes this.
In my testing so far,
Redline Monitor seems to offer the most neutral implementation - but that probably makes sense since it's a commercial product rather than a free plugin.
If you set the speaker position to 90°, leave the distance at 0m, and turn the phantom center down to about -1.8dB (that may depend on your headphone frequency response - I adjusted it when playing white noise and toggling the plugin on/off until there was no difference in volume) it produces a more subtle effect than most I have tried.
Redline Monitor offers enough controls to properly adjust how it sounds, but not so many to overwhelm you, like a lot of other plugins do. I could never get TB Isone to sound right, for example.
Configured this way, it still sounds like you are listening to headphones rather than trying to make them sound like speakers, but music is more natural sounding and less fatiguing.
There is still some high-frequency emphasis that most Crossfeed/HRTF has, but it doesn't make the music sound "thin" like a lot of the plugins I tried.
What I find interesting is that with a lot of music, the difference can be very subtle when you toggle the plugin on/off during playback - but there are some tracks which get considerably wider.
It seems like there must be some kind of spatial cues that are just completely lost due to the isolated sound going to each ear, which your brain only picks up on once it's enabled.
With most plugins, I always found myself turning them off with certain types of music. So far, I have been happy to just leave Redline Monitor enabled all the time.
There is still some music which sounds better with it off - well, not necessarily
better, but how I am used to it sounding - overall though, I think I prefer to leave it on.
The trial is a generous 60 days, so you have plenty of time to evaluate whether you like it or not. I find that most of the time I don't notice it being there - which is a good thing - but I
do notice when it's missing.
This video is also quite a good explanation and demonstration of crossfeed:
http://www.youtube.com/watch?v=v7iHJY-lbLE