Well, what are the options here?
1. M50 to M60
2. VP100 to VP150
3. QS4 to QS8
Are those the choices, or are there other things you are considering?
I have only heard my Axiom system, so I can't really give you more than hearsay. Sorry.
Personally, I went with the QS8's because I never wanted to wonder "what if?" and because I thought I would appreciate their slightly lower bass extension. I do not regret my choice, although I do not have the benefit of having compared them to the QS4's.
For HT ONLY (what is your music system like?), I don't think you would find much benefit in option #1. I do wonder about those that seem to have difficulty properly placing a VP150, so if you are currently "satisfied" with the center channel, I might not mess with it.
