"So my question is: is THIS tube amp really significantly better than this solid state amp?"
Its a difficult question to answer. You're the one that has to listen to it so it really comes down to what you think. Both are excellent amps but it looks like you only heard the MC5200, and not the 275.
You really owe it to yourself to listen to a 275 before you get one. If it will drive your speakers, chances are that you will like it. But there is no gaurantee. Its possible you may prefer the SS amp.
A couple of other posters recommend the Aesthetix preamp. I do too.