I've read countless threads on the "importance of mixing in mono" and I definitely understand the benefits of it.  That being said, I don't have an Auratone (or similar), so what I do to simulate this is put Ozone on my master buss and check "mono" on the Stereo Imaging module (I'm sure there are a million ways to do this).
 
Every time I do it, my electric guitars, which seemed pretty well mixed, leveled, and eq'ed in stereo, seem to almost vanish.  I just chalked it up to "it is what it is", as I got sorta similar results when I compared commercial mixes in mono.
 
However...I've been reading a lot about panning laws, and currently use X1's default, which I guess does NOT subtract 3db from the center channel?  Is that right?
 
I don't know ANYTHING about it, but I'm wondering if that could be the reason for my vanishing guitars (panned hard left and right)?  And maybe my "very busy center channel"?  I tend to mix LCR.
 
Any ideas are appreciated!  Thanks.

Are the hard-panned guitars out of phase with each other?

This isn't a pan-law issue, but rather a phase issue. You are probably using some tricks to widen the guitars, such as cloning a track and altering the phase relationship or adding a very short delay. Such techniques sound great in headphones but often fall apart in mono. If you want wide-sounding rhythm guitars, double-track them and use EQ to differentiate them, but avoid "stereo wideners" and other magic shortcuts.

Thanks for replying bit and gustabo.  Actually, I don't do the old "clone and slide a few ms" trick anymore.  What I usually do now is record one part hard left with a cleanish tone and another pass (different voicings) hard right with more gain/distortion/etc.  It's such a challenge to get them both to "be heard" in a mix...which is why I was trying to see what was going on in mono.
 
I was hoping that something other than my bad mixing knowledge was the cause of the guitars not punching through in mono.  That, and those pesky amp sims... (I'm already applying Craig's pointers from previous thread).
 
So much to learn!  Thanks again for the input.
 

Have you tried muting all the tracks except for the guitars and then listening in mono?  It would be interesting to know the result of that test.

Not yet konrad, but I'll check that out soon...unplugging for the upcoming holiday week...  =)