go for an american or british make. stay away from japanese brands here.
as a rule of thumb (to be taken with a grain of salt o' course) american amps present a bigger and more dynamic picture, while the british will be more 'musical' (there is a reason that all the fine studio stuff comes out of britan;).
I prefer the latter but better judge by your own ears.
also keep in mind that the nautilus series greatly benefits from bi-amping and good cables. so I'd rather advise to get 2 stereo amps for around 2.5k each than one stereo amp for 5k!
as for the american amps: pass labs is usually regarded as a very fine solution for b&w. definitely have look into these. also bryston are very good.
on the british side of things: try either atc or linn. atc makes very transparent and powerful amps. work very well with b&w imho.
linn is rather an offshoot here, but I recently was demoed nautilus 802d with linn chakra 4 channel amp (the 100 watt model - there is also a 200 watt 4 channel amps in this series) - simply stunning. fast and accurate, without being harsh, very precise imaging and more powerful than I expected.
btw: I run older 805s with atc SPA2-150. will demo a linn amp soon on them.