I use both the MC275 and the MC452 with my Summit Xs. There is a huge difference between the two. My room needs a lot of acoustic work (overly bright, I think), but of the two amps, I prefer the MC275. I like the big fat midrange the MC275 produces. I like the bass from this amp better too. Not as well defined as the MC452, but fatter and smoother, which I like. The high end is a little rolled off, which also helps in my bright room. I might feel differently after the room is treated.
I listen to primarily acoustic jazz at moderate levels. In a larger, better damped room, at higher volume levels, the MC452 might be the better choice.