I'm running a Seismic 12 with Signature S4's and it fills in quite nicely. I run it pretty hard in the 5.1 setup - it's incredible for movies but I turn the subwoofer level way down for 2 channel. Like most subs - if you push it too hard with music; you can over do it (IMO). Overall, I think it's reasonably musical. I moved it around quite a bit as well. I have spots in my room where it sounds pretty lousy and one spot in particular it really fills in the low frequencies. You might like it more due to the amount you will be gaining. Both the Signature S2's and the S4's (very similar design to the reference series) arent heavy bass speakers so the sub seems to fit right in.
In addition, I had to play with my crossover point for quite a while until I got it dialed in just right. I had planned on going to a Velodyne DD 15 but I'm so happy with the Seismic I decided to keep in ... in fact I'm thinking of adding two Seismic 10's in-line with the S4's and keep the Seismic 12 for my 5.1 setup.
I have seen The 12's go for $1000 and the 10's for $750 used. I figure by the time I'm done I will still have less into my subs than if I went with the DD-15 and I wont have to keep re-adjusting.