Can't agree more...
To me, it can't get much better than a DD-15. I own one, but I also own an SVS (albeit a medium sized one, not the monster you are talking about). They are two different animals.
The DD, as mentioned, is a servo sub that is in a sealed box. It can shake my room no problem, but I use it exclusively for two channel music now. Because of the built in digital EQ, which they are now selling separately, you can get the response to be very flat across the whole range. It is so phenomenally tight and accurate as only a servo can be, it is in a league by itself. I used it for HT before building a separate two channel setup, and it was unbelievable.
The SVS is a great HT sub, in my opinion. Its purpose in life is more tilted toward maximum SPL, as it's a ported sub, although it's decent for music. But there's no comparison in things like string bass between the two. So, if you want max SPL and good performance for movies, you can't beat SVS. But if you want the best music reproduction, the DD-15 is beyond reproach. The DD is relatively expensive, though, so that's always a factor.
That's my opinion.