Center channel - Need help

Center channel needs to match the front R&L speakers in tone. Or it will suck.
Pretty unlikely to be able to use another brand. But possible to use another series?
IMO one major factor in choosing speakers for 5.1 is the middle speaker matches the front R&L. Even if the front R&L are going to be two channel use also.

A third speaker identical to your left and right channels would be ideal.

I think a used Gallo AV Reference Center ($600-$800) would be a great match both in terms of sound and style. Or, a Gallo Strada (less LF coverage but extremely clear, open and dynamic).

Not really a B&W fan, but I have to say one of the better centers I've heard was the Matrix HTM (the one with the single midrange driver). It was very clear and coherent with none of the combing effects found in some dual mid configurations, so voices sounded consistent no matter where you sat in the room. There's one available here now for $300 that appears to be in good condition (no affiliation). Anyway, best of luck.