Hi BruceH,
I currently use M22ti' s, a VP100, QS8s and an EP500 sub. The M22ti's are on an A/B switcher so I can instantly switch to a pair of M80ti's up front. In my modest living room (2,100 cu ft.), running in 5.1, the M22ti's and M80ti's are extremely close, so close that I have to look at the switch box to see whether A or B is running.
I used to use a pair of Mirage M3 bipolar floorstanding speakers in a stereo configuration with another A/B switcher that went to a pair of Paradigm Reference Studio 20s running with a subwoofer.
Overall, I agree with you that assuming you have neutral well-designed main speakers and a similarly neutral center, you can mix brands. I'd disagree with the audio shop guys who said it doesn't make much difference. I've detected really annoying coloration with different brands of centers that I've substituted for the VP100 or VP150.
To my ear, it shows up very quickly on dialog or vocals, male or female, because our hearing is so sensitive to midrange sounds.
If you are going to mix brands, the surrounds are the least critical. Or rather, you're not as aware of crummy surround speakers unless you concentrate on their flaws and remove your attention from the movie and front soundstage.
Regards,
