As far as the multiple subs go, ideally you'd want the same subs. If not, it's not a giant problem. I run three subs--one 12" ported, and two 10" sealed. Probably not the greatest mix, but it's what I have. Having three really evens out the response in my room as well as making the low frequencies less localizable.

If HT is your thing, I wouldn't go smaller than 10". The smaller you go, the less extension you'll have.

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose