Whenever I am listening to a CD and switch from stereo to surround, I find that the surround modes wash the sound out and it doesn't sound as full as when in stereo. This is a function of the sound processing as a opposed to the speaker combo.
Using matched speakers is the easiest way to get a good sound from your HT system but using a combination of different brands will work as long as they are matched in sound level, power and frequency response. Personally I like the matched set.
Choosing a Subs would be the exception to this since their frequency range is outside of the rest of the system.
