Best Small Sub

I have been working on building a setup for 50/50 music to HT. I am only running a 5.1 system. I have Dynaudio Audience 52s (LCR) and 42s (SLSR). I have a Halo A52 to power all the speakers. Currently I am using a Denon 3805 as a pre/probut will be upgrading that later on. I want to buy a sub next but it needs to be a small footprint. I dont really have any room in my 15X17 room with 15 foot ceilings. My seating arrangment has the LCR and TV 8 feet in front so it is very overpowering. I know it would be best if I used a large sub but would rather start with one small then buy another later on if i can make room. Can you offer any help of subs that would really sound great and mesh well with my system? I was looking at a sunfire and velodyne SPL. Any help would be greatly appreciated.