Many years ago a friend of mine had the B&W Matrix series speakers in a HT setup, and even that relatively lower-end center speaker in that system sounded awesome. Completely uncolored, not boxy, and with excellent dispersion and uniformity across the room. I think one of the most important reasons for this is that they mount the tweeter above the mids (or midwoof) instead of between them as most models do. Personally, I’d never buy a center channel unless the tweeter is mounted above the mids.
If I’m you, I’d buy the best B&W L/R/C you can afford that are of the same series so there’s a seamless match across the front. As for the amp situation, given your amp’s power I’m not sure you need to biamp at all, especially if you’re using a subwoofer (or hopefully 2). I’d just buy the speakers and try driving them all with a single channel each and see what you think (or perhaps even try bi-wiring your front speakers instead of biamping them to see what that does if anything). My hunch is you’ll be perfectly happy given the power reserves of your amp, but if not you can add a supplemental amp later. Anyway, hope this helps and best of luck!