Here goes:

My current setup includes a Single 12" Polk Subwoofer powered by the second channel of my amp in bridged mode. The Sub currently sits in a q-logic custom fit enclosure. As it stands the specs for the speaker and the box work perfectly (i.e. speaker calls for .85 cubic feet, enclosure volume is .85 cubic feet.)

Recently I found this MTX Thunderforms enclosure at Crutchfield. Now normally I'd move right past this item were it not for the two 10" Polk GNX subs I have that were part of my previous vehicle's installation (their enclosure wouldn't fit in the truck. ) I ran these two bridged and in parallel and to be honest they sounded amazing!

The specs of the MTX enclosure are 1.77 cubic feet. My 10" subs call for .66 each (1.32 cubic feet.) Would the fact that this enclosure is a little bigger ruin the sound? I like the idea of getting a little more room from the back but don't want to sacrafice empeg/audio quality to get it.

I think the shape and materials of the enclosure, as well as whether the enclosure were ported or sealed, would affect the sound to a greater degree than a few cubic inches of space. I think you're fine.

I also like my Polk sub in its Q-logic box. Although it's a standard trunk box, not one of those custom-fit jobs.

Good questions .

I'm just guessing but I'd suspect that Polk specs include the speaker displacements and the enclosure's specs do not. I'll call around and see what I can find.

there are alot of sites where you can plug in those displacement values and get the optimal enclosure volume.