Center and Mains

IMHO, the biggest problem is the extremely bad match between the center and main speakers. Having different brands absolutely ruins any chance of timbre match and then there is a 6db sensitivity difference between the Def-Techs and the McIntosh.

Sounds in the front may come from a single speaker, or they may be created with L&C or C&R speakers, and that is a problem with the mismatch.

Read this article, especially the last line.

http://www.avtruths.com/speaker.html

You can change the speakers to match. Here is the matching center for your L&R that is recommended by Def-Tech,

http://www.definitivetech.com/products/cs-8080hd

An amplifier will do nothing to overcome the timbre mismatch.

In a 2 channel system, the speakers create a balanced image with sounds that come from anywhere in between the 2 speakers and even outside the speakers. Now if the 2 speakers were different, that image would be totally ruined.

In a multi-channel system, that perfect front image can be created with 3 identical speakers, however that is only possible in certain situations. The next best option is to use the timbre matched center that was designed by the company to go with the L&R. If seating is tightly centered between the L&R, the "phantom center" that was mentioned can work well in some cases. Any other center speakers will just degrade the front sound, and IMHO, trying anything with amps, processors, etc is a waste of time. Doing this will always make it sound different, but never right, and your ears know that. When it's right, your ears won't tell you to "clean up" the sound.