I have listened to many subs and there are only a few that impressed me with music reproduction, the ULS 15 is my favorite for the money {actually my favorite I've ever heard}, the svs sb12s are also great, and I own a pair, but they don't have the output the uls15's have and the hsu just seems to have a little something extra, an effortless projection, kind of like nothing is there making the bass.. hard to explain but its an impressive sub, you wont find someone who can honestly say a bad thing about them... I bought the sb12's because I wanted piano, they are half the price {I got a great deal on them}, and I didn't need the extra output....
I have a good amount of experience with the uls15's, I put together a system for a friend of mine, similar to what I listed above except ascend sierra towers, and an xpa2 amplifier. The system sounds amazing, he has it set up in a 400 sq foot room and it sounds like a concert when he turns it up, but crystal clear, balanced, and strong...
A lot of guys say you don't need more power, but I have tried it, I am cheap so if I could get away with a 100watt receiver vs a $1000 amplifier then I would buy the receiver and save the money, but when trying to use lower powered electronics, I clipped, I could tell the sq suffered, and I will never go back.. I do however believe that most modern amps {and electronics in general }sound so close to each other its not worth spending the extra money for a name, I say it all the time, if you have $3000 for a stereo receiver or integrated why not get a pair of xpa1's and an xsp1 or xda2? They will cost less have more power and in most cases have better customer support and sound better with more features, not to mention resale value, if you spend $3000 with emotiva or outlaw, ect you can sell that stuff in a few years for 90% of your money back, where with some other gear you would be lucky to get half....