Sorry I don't know if this allowed or not but heres a link to AVSForum to a thread that was a sub shootout that directly compares the ULS-15 to the XS15.
NE PA/NY/NJ/DE/MD/VA GTG - Take Two! This thread is what got me pointed in the direction of PSA. I originally wanted to go with dual ULS-15s. Then I started reading about the XS15 and was going to do with two of those. But after some back and forth with Tom he said to just hold up and wait for a new sub which he figured I'd love.
That sub turned out to be the XS30 and I've since ordered two of them and they can't come soon enough!
I guess to give you the short version of the thread; the XS15 was just as good as the ULS-15 and was cheaper. But I will say there was a small noted problem with the ULS-15. But they still used it in the shootout. So if the XS15 can hang with the ULS-15 in theory a XS30 should blow it out of the water. And is quite a bit cheaper too.