Where to go from Rogue M-150 amps?

I'm curious the hear the responses and opinions on this question. What would a be a good upgrade from the M-150s. I'm not really interested in the factory upgrade to the M-180. I'm currently using my 150s with the Perseus preamp and Vandersteen speakers. Overall, I'm very pleased with them. Great bass authority and control. The mid are good, but could be a little more fleshed out and intimate IMO. The highs and detail is there; however, more air on the top end would be a plus. I'd like to stay tube, but will consider SS. Monoblocks are desired but not an absolute must. The amps must work well with Vandersteens as they are my speaker of choice followed by Magnepan; but I will probably stay with the Vandersteen line for a while.

Thanks!