I've heard the M60 and didn't hear enough of a difference to think it was worth it over the M22. I am interested in what I hear about the different sound of the M50 though, and think that I would like it. My room is 18x12 upstairs.

No, the downstairs isn't for critical listening, so I could go with whatever down there. Moving down the M22 just gave me a reason to get the M50