I second the Plinius 103 recommendation. It has plenty of power for your speakers, and it will add a nice dose of refinement and overall improvement with its Class A sound. What's really nice about the Plinius in particular for your situation is that it has a low/high bias switch (not available on Pass Labs as far as I know) so when doing home theater/non-critical listening you can switch to low bias so amp runs cooler and more efficiently. The Classe amps are obviously another good choice as they have synergy with B&W, and yet another would be a McCormack DNA500 if one becomes available. I don't think think you can go wrong and will be very happy with any of these.
One other dark horse you might want to consider. I had a Liberty Audio B2B-100 on demo in my system and found it to be a truly excellent amp. Very detailed yet refined (especially in high-bias mode) with outstanding imaging and soundstaging. Supposedly sounds even better in monobloc form and still under your budget new. A further benefit is I think Peter (same designer as highly regarded PBN Audio amps - this is his consumer direct channel) would let you demo them, which is so critical before buying.
The other thing I'd HIGHLY recommend looking into before doing anything else is a separate stereo preamp. The Marantz is good for a prepro, but with your level of equipment I think you'd hear a big improvement with a dedicated high-end stereo preamp, especially after upgrading your amp. It can easily be integrated into your current system such that the Marantz can be used for home theater and surround music duties, but it is completely removed from the signal path when doing critical 2-channel listening.
Sorry for the long response, but hope this helps and best of luck in your search.
