I'll answer your last question 1st.
What would I do...I bought two of the HSU vtf 3mk5 subs that a few others are suggesting.
View attachment 29352
Now circling back to your earlier thoughts.
Granted, a good CC or LCR speaker combination raises movie audio quality by leaps and bounds over the TV speakers, but its the sub or subs that put the
theater into the HT experience.
Tempting with your budget, but I would suggest not to do it...don't buy one larger sub and call it a day or come back a yr or two later and add another one. Even with one killer sub in a room the size you have, without perfect placement, not every seat in the space is going to experience the bass like it was intended to be. My space is 18x17x9 and open to large open spaces...these two babies cruise along until they command the stage and then they can rock the house when called upon to do so.
All of the brands mentioned are going to perform well for you...from a value stand point, it's hard to beat HSU.
Good luck with it.