Definitely replace the kube.
Like 2ch said. Multiple subs is more about multiple seats and if you have freedom of placement you can definitely get good results with a single. Depending on room size, I’d recommend a ported sub. The SB2k is better than the kube in every way possible, but if you have a big room it will struggle on big films(but still be better than the kube in that regard).
FWIW, the pc/pb2000 is in the outlet for 699.99. I think they’re still in there anyway.