Originally Posted by csgamer
Because horizontal centers are a compromise to save on height and suffer from lobing, comb filtering, and worse imaging. The one benefit the q600 might have compared to the q300 is output and saving on height if placing the q300 vertical, but that's it. I would take the q300 over it for center channel duty. I'm in the minority on AVS, everybody loves their horizontal centers here.
However, I will say that the q600 is a much better design than the typical MTM horizontal center.
Normally I would agree with csgamer on this - if feasible loose the horizontal center and get either three identical speakers LCR, or, use the matching vertical bookshelf speaker as a center.
However, with Q900's as LR mains this situation is one of the few where I might disagree and recommend the (horizontal) Q600 over a Q300 as a center. The Q900s are huge and output a lot of sound, I would worry a single Q300 might not be able to keep up as well as the Q600. And as csgamer mentioned, the Q600 is "a much better design than the typical MTM horizontal center". (To my ears the concentric tweeter/midrange driver reduces lobbing compared to the other MTM horizontal speakers I've heard.)
If two-channel stereo music listening is the priority I would recommend Q900's with the Q600c.
If HT/surround is the priority I would whole heartedly second csgamers earlier recommendation of 5x Q300's all the way around.
Re different speaker brand's sq ... there are a lot of good choices and personal preference really comes into play. At this price point you probably should consider auditioning your top two (or three) choices in your home. (You don't need to audition a full 5.0 set, a pair will do.)