Does anyone have a suggestion or advice ?
If your primary focus is on movies, then a ported sub is probably a better choice -- it will give you higher SPL and greater extension.
SVS is always a good choice. If going ported, then I'd consider Hsu Research as well.
I've owned SVS and currently own a pair of HSU ULS-15.
Flatsmaster, you may have already made your purchase. I haven't been on Audiogon for a while. Since your loudspeakers are B&W, then keep it simple and don't get overwhelmed with the plethora of subwoofers on the market. Stick with B&W. That's what I have done and they work very well together. I actualy have two in my system for improved and more balanced room loading. Your room is large and going with two subs placed apart from each orther would be the way to go. The only catch is that you're spend more than $500.00 for both new and possibly even preowned. But in the long term, it'll be well worth it.