I own the v2 of the Paradigm Servo. (Grilles only off for pic)
No doubt the Ultra 13 will be an awesome sub, with it's 10Hz tuning port!
I have constantly been blown away by the Servo 15's depths and precision. Listening to bass monster tracks like Underworld's "River of Bass" and Happy Monday's "WFL Think About The Future" I just hear and feel the bass like I've never experienced before. Movies and Hi-rez music are also an awesome experience.
That said the Ultra/2 is also an awesome sub - it's 190 lbs!!! The Servo still maintains less THD when you get down to the depths, but Home Theatre Hifi measure only 15% THD at 100dB with the Ultra which is pretty sweet. The Servo flows quite smoothly down at that depth and does not go above 8%. However HTH and AV's conditions are totally different, so those results can really be compared.
If you want a serious sub and you're considering either the SVS Ultra/2 or the Paradigm Servo v2 I really don't think you can go wrong. I didn't.