daj,
First, turn OFF all the Denon's auto-EQ and manual EQ. These circuits are crude at best, and may degrade overall fidelity of linear speakers like your Axioms. I suspect that is what's happened. Try to get your center and mains sounding smooth, neutral and agreeable before you fool around with EQ adjustments. Using a pink-noise test signal, the center will never absolutely match the left and right mains, but when it's carefully placed, it should be close enough that there will be an integrated front soundstage with movies or music and no distracting coloration on dialog or vocalists.
The center placement is very critical in lots of rooms (most). The less it is encumbered by cabinetry, including your TV as well as other furniture and surfaces in the room, the better. Try putting it below your Mitsubishi screen on a temporary stand. That in fact is where I use my own center and where it sounds most neutral.
Do not bury it at the back of your Mitsubishi. You may want to try wall-mounting it behind the set but you shouldn't have to.
The internal speakers of some large screen sets have improved, but they are no match for a well designed and placed dedicated center channel speaker. The speakers in my Samsung are decent and adequate for casual viewing of shows when I don't want to fire up the whole 5.1 system, but by comparison to the Axioms, the Samsung speakers are quite colored.
Regards,
