"The Velodyne HGS series have the lowest distortion and flattest frequency response of any subwoofer"
There's a Swedish engineer with a subwoofer claiming 140db dynamic range--they say its "the best in the world." I'm sure it could hold its own with Velodyne. There weren't many specifics in the article since its focused on the new digital formats. But, I'm sure its distortion is quite low, especially when brought to a 'meager' 110db or 120db. The Velodynes aren't bad, but there is better. And I still wonder what the performance of the Kharma Acoustics Grand Enigma really was.
"I have a ULD Series II 18, which crosses over to my Budge
Khorus speakers at 60 hz. It does not sound slow or on-musical."
I would just like to point out with subs that there is a distinction between the actual speed and perceived speed (subjective, "it sounds fast"). I'm not qualified to comment too heavily on the difference, but in reality, cabinet colorations and internal standing waves, a non well damped cabinet can muddy the sound regardless of what the driver is physically doing: whether or not it is servo controlled, etc. Not to mention dreaded room acoustics coloring things up.
[The crux of servo control from what I gather, is that since it is a measuring device there is technically a time-delay between sensing the drivers motion and then sending the appropriate command to the circuitry to correct the motion--this is why some bash on it. It may not be significant given the human ears perception down low isn't as sensitive as the midband region, hence the benefits to the design outweighing the possible negatives (however, the switching distortion in a Class B circuit is still far shorter and the Class A buffs complain of that)]
Audition them both (all) if you can, but be careful of room room placement and nodes. Do your best to level the playing field. Velodyne are nice subs though and not a "home theater" sub (at least not the better ones). Ideally a sub would do it all, but when you have to compromise the design, the differences for a HT sub would come when the designer opts for a higher system Q, higher output, which in turn usually means greater efficiency going the bass reflex route and less extension. A comparably priced "Music" sub would possibly have a lower Q and less output (maybe sealed), with a bit better extension. (Note: I am not encouraging the "Bass Reflex=HT and Acoustic suspension=music" stereotype. I'm just using an example. There are very nice ported systems, along with many other possible designs: TL, etc.)