I'm proud to have guessed your ratio correctly. Yes. Many do favor the M60s. That's what pulled me to them. This forum. Otherwise I'd have a pair of M80s myself. But I was mostly turned from them from a power/room size standpoint (which you seem to lack in neither).
I've never heard the 50s...but from what I've read on the forums they make a better rock music speaker. Perhaps the 50s are a good fit afterall. How much volume they produce, I'm not sure. But as far as style to perfomance they'd fit really well. I'm sure they'd do great for movies as well.
So, I suppose for you musical taste I'd go for 50s. For movies...I'm sure you'd notice a difference, but how much is uncertain. You'd lose some low end, and certainly the high end (one less tweeter per speaker)...I guess you just won't know till you try it. Hopefully somewhere out here in the great Axiom forum is a M50 user waiting to give you his thoughts...
Good luck!
