The Aerials are somewhat inefficient (86dB, I believe), so if you listen at high levels or to very dynamic music you may find the MC275 somewhat lacking. That said, I have a C2300 that I use with either MC501s or a MC275, playing through 87 dB speakers (Magico Mini IIs). For most of my listening I find both amplifiers equally enjoyable. The bass with the MC275 is a little looser, but the midrange is very sweet. When I switch from one amp to the other I do not notice huge differences otherwise. I am not sure I would recommend switching from MC402 to MC275 as an upgrade with the expectation of a big improvement (or even difference) in sound. But as an alternative, if you can afford both I think it is worthwhile. I use tubes in the winter and the SS amps in the summer.
Caveat - I do not listen at concert levels and I have old man ears so frequencies above 12kHz don't mean much to me. Mostly listen to acoustic music, chamber, jazz and classic rock.
Mike