Thanks guys.
Sana I've known this before I might have tried using two big center instead of a bookshelf.
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You know I have an old receiver (Yamaha) with two speakers out as a center speakers.
By the way, since most of center speakers have two woofers it doesn't mean you don't need sub woofers?
In my opinion kaya most of the center speakers have two woofers first it easy to balance the speaker as center, this should spread symmetrically but there are cases that only one woofer like the kef & sonus solo, but their tweeter is on top of them which has more complicated cabinets.
2nd center speakers are really made to match floor stander speakers & bookshelf are for sorround
There are speakers designed the same with the center like the THIEL CS series. They are D'Apolito principle, two woofer one center tweeter which can be place vertical or horizontal.
just my personal opinion