I really like tower speakers but since you are satisfied with your volume levels you may just need a subwoofer.
I have an Outlaw LFM1-EX and I am very pleased with it.
This is my first subwoofer and when I bought it I was very unsure about whether I needed one or if I even wanted one in the house.
After owning it for about a year I can say with certainty that I needed it and it has also been very easy to live with.
The gear you have is good and your room is fairly large so the LFM1-EX is what I would consider the entry level sub for you.
SVS, Hsu, Rythmik, and Chase all make a nice product.
If you just want to go with towers I think listening to them and picking what you like is a lot better than buying from recommendations and reviews.