I have two MC275's mk.V's running mono to power a set of Harbeth SHL5's and I love the sound. A truly epic experience to say the least! That being said, I am considering a switch to the MC501's for two main reasons, life style and a tighter control of larger speakers. The fact that I won't have to re-tube down the road and the fact that I can leave them on without worrying about degrading the life of the tubes is a definite plus on the S/S side. Also, with inefficient speakers like the SHL5's, 350 more watts of power would roughly equal 4db more dynamic headroom, which should help a great deal in controlling the drivers.
I have heard the MC501's and thought them to be fantastic, and think that it would be more of a switch of attributes rather than an overall loss. Nothing will sound like a tube amp other than another tube amp, but that being said, It is hard to match the sonic benefits of S/S amps with a tube amp. The Layering and dimensionality of the MC275 makes it one of the best amps around period, but the control and accuracy of the MC501's make them also one of the best amps around period. At the end of the day, it will up to a number of other factors that will help you finalize your decision, but I don't think you or I for that matter could go wrong with either one.