You could put the center either above or below the screen (or one above and one below for you overachievers). Folks seem to think that the horizontal dispersion from a vertical center is better than any horizontally-oriented center channel.
BUT I think one of the big advantages of a vertical center is to have three identical speakers across the front. You don't sound like you want to replace your mains, which is fine. I've really liked all the DCM speakers I've ever heard.
If I was going to have a "dream home theatre" though, I'd get identical speakers for the L/C/R.
If you keep the DCM's, you're probably not going to get an exact timbrel match between them and the center channel. And that's okay, because even center channels from the same family (like Axioms) tend to sound a little different than their vertically-oriented brothers due to speaker placement and boundary effects and so forth.
Of course, most people aren't willing to make the aesthetic or design compromises necessary for a vertical center channel. I think that's okay, too. I'm really perfectly happy with my M60/VP100 setup, but I do recognize that designers have to work pretty hard on crossover baffle step correction on horizontal center channels, which can result in some tradeoffs you don't have to make on vertical speakers.
I think there is consensus around here that the VP180 is a pretty big upgrade over the VP150. I haven't personally heard a VP180, but I trust those forum members who have made the direct comparison in their homes.
I love my QS8's. I think they are terrific. I haven't heard your other surround candidates.
Have fun.
