Here is a good read for you,
http://www.soundandvision.com/content/matching-front-and-surround-speakers
A HT will always sound best with timbre matched speakers, and the perfect timbre match is all identical speakers (the same brand and model), however this is not always possible. As the article states, some receivers or preamp processors can change the sound of speakers, and though this may help, I find it very hard to believe they can make a Paradigm sound exactly like a Def Tech.