I strongly suggest you look at the Cary Cinema 5. The Absolute Sound's multi-channel amp of the year award. Cary is known for tubes and they carry that warm, analog sound into the Cinema 5 which is a solid state amp. However, the warmth does not obscure the transparency or dynamics. Plus, B&W's love power and the Sig 805 will appreciate the 200W (Class AB)x 5. Actually, the true power rating is about 225 but Cary rates their amps conservatively. I use the Cinema 5 with B&W 803's and they make them sing!
Read the review on Cary's website as well as the comments on this site regarding this amp. Good luck!
AVGURU