Generally, any of the Ascends would be ok for center use, IMO. Obviously, the S1 center is the same as your S1's. You could probably use the CBM on its side with your CBMs. All depends on your budget.

I've gone from S2 center (with my S2's and then Towers) to Duo center (which was a step up over the S2 IMO) to eventually a Horizon RAAL (ordered and waiting somewhat patiently ;-) ).

I find movies are still loud then soft even with a center - it's how they are mixed. So adding a center likely won't resolve that issue, but (IMO) having a center makes for a better movie experience with the dialog firmly centered, effects, etc.