I would not say it was routine, but not unusual to mix brands of subs. I have a M&K V-1250 as part of the 750THX system and am considering it myself. Should have no effect on the quality of bass just how it is transmitted throughout the room.
Totally different animal than mixing your front three which should be very close if not identical.
With multi-subs often have better balanced ouptut response as the result of levellng out some nulls and peaks.


[This message has been edited by DaleB (edited November 30, 2003).]