Irv, this is really difficult to fathom. You're comparing two sealed subwoofers, and you think one sounds substantially cleaner and more musical than the other? Since both measure with reasonably flat frequency response and low distortion, why do you think that is the case?
When we were comparing the svs, the song that showed me the difference was Adele's Rolling in the Deep and while the bass from the svs could easily overpower the room, the uls had this SMOOTH ULTRA FAST sound, the bass was so bouncy and you could almost bat it around the room, lol... glen {used to own quest audio} said it was because the room was "better" suited for the uls, but I find that hard to believe, why would one sub sound good in the room and the other not sound as good, BUT there has to be a sub you prefer, obviously... You have to listen to the uls15 to know what i am talking about, normally i prefer smaller sealed subs for music but what ever they did when building the uls15 was rite... There are obviously better subs out there, and I like svs products too, I just bought a pair of sb12's for my self. But for the money I prefer the uls15 to the sb13.. I felt like the sb13 had really good output but it had rumble in its musical bass where the uls had smooth reproduction, like a distinct start and stop vs a rumble to stop... Sorry if I am not explaining this good, its hard to convey what the differences are....
You will have a better idea if you play the adele song, turn the bass up and you will hear what I am talking about, there is this strong bouncy bass in there, that some subs don't react well to, I know it will show you the difference between a ported sub and sealed for sure....
http://www.youtube.com/watch?v=rYEDA3JcQqw with the uls15 and the svs sb12 this song sounds awesome, with my sw300's, the sb13, kef 9", and a couple others I have demo'd it rattles it around a little.... This is one of my test songs, and the bass isnt super low but it separates the good from the great... Also the mids and highs are really nice in that song, I love her voice, this is also one of the songs I use to show off my systems, people are always floored by how revealing it can be...
Don't get me wrong, the svs is an amazing subwoofer, and you will not find anything as nice for its price with that kind of output, for a sealed sub that thing smashes, it is LOUD... But I prefer the uls15 after spending some time with it.... On the other hand I like how the sb12 sounds a lot and I think for music where you don't need 3600 watts that sub for $600 is a great bargain...