Thank you that's also great info. I am using REW to keep improving the room and setup especially after improving on treatment. I have DIY absorbers and combo panels placed, I am having a tough time with bass absorption though due to room dimension.i had the same concerns about music and price, so i got a DS1510 with H600PEQ3 amp, and stuck it into an old hsu sonosub that i had laying around, but i sealed it, instead of venting: http://www.rythmikaudio.com/DS1500ci.html
rythmic audio uses servo feedback from the sub, that tells the amp the position of the cone, it works really well for music... i used rew and a mic for optimal placement in the room, which made a big difference, including huge gain on the very low end, in part because it's a pretty small room.
i would suggest that if you haven't used rew or something similar before, do that before anything else, because you'll definitely need it for adding a second sub... room treatment would also be a good idea, you can measure the difference it makes with rew.