Considering trading in the 180 for a 160. ... Decisions decisions.
The QS10's are excellent ... I have them in a 5.1 setup (w/ M2 fronts, VP150ow center, and EP500) and I have not been wanting as far as sound goes (considering the room and it's function). It's worth stretching a bit.
If the answer to the next question is only a difference of $$ then the decision is easy:
What does the 180 give you that the 160 does not ... and do you need it?
(The reverse is also true ...)
It seems that all the V4 centers are pretty good (although I can't speak for the VP100). I have the 180 and would not hesitate to recommend it. Given the high praise of the 160 by people I trust here I would not hesitate to get that either. The VP150 get's diss'ed here a lot but I'm convinced that's because everyone is referring to pre V4 versions .... the 150V4 on-wall, in the 5.1 setup above, is a fantastic match with the M2's. I'd recommend that as well.
Besides the money, I think the center decision comes down to the size of your screen and the size of the room you want to fill. The 180 is huge and I'm pretty sure you would not want it under a 40" TV. It's an excellent choice for something like a projection screen. The 160 has a vertical requirement to consider.
The 150 in my mind fits visually with most tv's today. IMO the on-wall versions are the way to go if you care about the visual appearance or having any space to place your equipment on. So far, I've been very impressed by the on-wall versions of Axiom speakers ...