Hey Y'all,
I have about the same room set up as you, Garyb6, and I have two Strata IIIs. They are just what the doctor ordered. My room is 22'L x 12.5'w x 9'h and the back of the room, opposite the system, is open to the foyer. I've gone through three other subs, B&W ASW-500, no big deal, Velodyne FSX-12, better, and the Rel Q100. Every time I made a change, the system sounded better but there was something missing. After reading the recommendations hear on the 'Gon, I decided to try two subs. The result is amazing! First off, no standing waves, the opposing subs cancel them out. Second, you have two drivers and two amps doing the work of one. I've got all the output I can handle in a room that is pretty big and they are still as musical as all get out. I also have both of them set up behind my system in opposite corners; I was told, by a rep at Rel, that this is the best way to get the most from these subs. If you check the instructions, it will give you that as well. It is time consuming and tedious but, in the end, very much worth the work. Sugarbrie is correct that these subs do not go down enough to match well with a full range speaker in the future. But, if you get the second sub and set them up right, you won't ever want for full range speaker.
As for under the sub, I replaced the original spikes with Audiopoints on both subs and it tightened up the bass very nicely. I've also had great success with the Shunyata Diamondback power cords. They are not too expensive and you will hear an improvement.......John