I recently replaced my Bryston 7Bst's with Pass X600's. The Pass monoblocks are a good match because they're very true to the music with precision and accuracy in the midrange and treble. The Watt Puppy 6's are particularly revealing in the midrange and treble. The X600s also have a smoothness and liquidity in the midrange, dare I say tube-like, where the WP6's tend to be a little hotter, and they are a good, complementary match here because of this.
At the low end, the Brystons seemed to provide a little more slam, although the X600's are not deficient and generate plenty of bass. I don't think the WP6's need any extra help down to the mid to low bass anyhow. (Since they don't really make it to the low low bass, I have a pair of self-powered Sunfire True Sub Signatures filling out the bottom octave.)
I think the X600's are a good match for the WP6's. None of the points I noted is a huge factor; some of them are fairly subtle and subject to my own tastes and interpretations. The 600 wpc X600's have much more than enough power for the relatively efficient (94 db sensitivity) WP6's, so this would suggest that some of the lower powered Pass amps might be a good match also, which could put it in your budget. Of course, I recommend trying them out in your system if possible.
On other threads, I've seen strong recommendations for various tube gear, especially Atma Sphere OTL monoblocks. I'm not a tube guy, but this sounds worth exploring if you're willing to go with glass. Good luck!
P.S., You know of course that Wilson is replacing the WP6's with WP7's with a roll out at the end of this month at the Stereophile Show in NYC. Reportedly, the WP7 is a completely redesigned speaker with new enclosure materials, driver set, crossover parameters, etc. Presumably, it will sound different (gotta be better?), and the amp synergies I mentioned may go out the window for the new WP. You have a nice day now.
