I also have an MC275 and a sub. Rel Storm III in my case. Speakers are B&W 804S. Sounds great to me.
Lately I've been focusing on improving my digital front end, but one aspect I have toyed for a while is relieving the MC275 from reproducing low frequencies. Even if your speakers can't get that low, your amp is still bending head over backwards to reproduce the low end, which is consuming significant power and where tube amps are not best. So, introducing a crossover between the pre and amp and allowing the MC275 only to amplify say 100 Hz and up, while letting the sub amplify from 100 Hz and down looks interesting on paper. This is called biamplification and some people swear by it. I haven't tried it (yet), but thought I should bring this to your attention considering the options you are considering. Please note not all subs can reproduce 90-100 Hz well - my REL for example is not good above 60-65 Hz. Even though it's capable of doing higher fdrequencies, it doesn't sound good.
I hope this is useful.