So I am curious... why so stuck on the speakers? I have never been a big fan of b&w... and while I own a lot of vintage gear (and appreciate their sins of omission) I think the Sonic Frontiers stuff is very musical... and not in the least cold (room playing a heavy factor here... never mind personal preference).
My advice would be to dump the b&w's. I also think things like digital eq and active biamping may generate a lot more benefit than changing pre/amp (then again, if you just want new gear... that is another topic :-).