What 2 channel Amp would you Buy for $6k?

Classe and McIntosh are classic pairings with B&W, I don't think you can go wrong with either one. I do think you are on the right track, adding more power to your system. I've owned a lot of McIntosh but not the MC452 so I can't say exactly how that would be, I can only add that for the same dollars you are looking at spending (or even less), you can buy a pair of MC501's (that's me recommending stuff I own). The great thing about the McIntosh is that you won't lose any money on resale if you try it and don't like it. The Classe will hold it's value too if you get it at a rock bottom price. I have only heard Simaudio gear with Dynaudio speakers and can tell you it was impressive.

Classe is closing its doors...may affect it resale value
McIntosh on the other hand works very well with B&W and has a steady resale value

What would anyone pay for a brand new McIntosh Mc501 Power Amp? Is $5k too much?

If your burnt out from your research maybe try approach from the other end.  Are there any speakers you particularly like (sonically)? Here in England (UK), there are many who like the ATC speakers and will seek out amplifiers which would go well in synergy with the speakers in order to get that sound (glorious midrange)...  Its just another approach.