I built my system in a bit of a weird way, I had no intention of going AV when I bought my BR5s, I bought them for a stereo setup. I then tried to fill out the AV package about a year later when I ditched my Marantz for the Onkyo. So to answer your question, the BR centre was not available in walnut when I came to buy, but have been informed by a few people since then that the BX was a significant improvement anyway. So dumb luck really!
I only compared the T3 and the Monitor Audio BR. The T3 makes alot of noise for its compact size, is a steal at £250 and is very musical with the high level connector. It blends perfectly with the BRs. If you have a big room to fill or like shaking the house down, there are bigger subs you can buy for the same money!