Andrew, I also like the tight, punchy bass that is very well articulated. My room is exactly the same size as yours and at one time I owned dual PSA S1500's. One was plenty, but I added a 2nd to smooth things out more. However, another sub that you should consider is the Hsu ULS 15 MK2. It is very close to the S1500, but cost a bit less. Had one of them at one time as well. As far as the S1801 goes? I do not have any direct experience with it. But, knowing Tom V. it would be a stellar performer as well. Here is a link for the ULS 15 MK2:
http://www.hsuresearch.com/products/uls-15mk2.html
At times, you can get a B-stock ULS 15 MK2 for only $762 shipped! Mine was and I never found anything wrong with it. Both of my PSA S1500's were B-stocks as well. You would be hard pressed to go wrong w/any of these mentioned inmho. If you are interested in the ULS 15 MK2, then give
Kevin a call as he will be more than happy to assist you. Best wishes!
Cheers,
Phil