I was off reading about the best placement for subs in a car, and the opinion of most is in the trunk, not in the 'cabin', but I haven't been able to find a clear explanation of why. I did try the Kenwood sitting on the package shelf, and it wasn't 1/2 as good as in the trunk. I think it's because the sound waves cancel each other out when the unit is in the same enclosed space as the listener.
Here's a link to a good study of mounting options in the trunk
http://www.installer.com/tech/aiming.html
Well, on the idea of combining the Infinity with the 2 MTX's, it's still an experiment. I've had lots of back and forth with the car audiophile crowd on this. My theory is that if all three cones are moving together, as in 'in phase', I'll get more SPL since I'm moving more air. Combined, I'm running 450W RMS, no big deal really, with 3 10" active speakers and 1 passive radiator. All technobabble aside, it just sounds better, with more punch, with the Basslink than without.
Here's a link to my rod site, go to the sound system pages:
http://bryceandbernie.homestead.com/BrycesRodWebpage.html