Here is Pete, who works for HSU response to the distortion in question from the HSU Forum:
I'd like to thank Ross Jones and Dr. John Johnson for the great review on the ULS-15 and HB-1 MK2/HC-1 MK2.
Some of you may be wondering why THD was 15% @ 20Hz @ 100db with mic 1 ft away from the cone. In Dr. Johnson's room, at the location where the ULS was placed, notice from the frequency response graph at 1 ft distance that there is a deep bass suckout where 20Hz is 6db down vs 60Hz. This inflates the 3rd order harmonic with a 20Hz test tone, which is what caused THD to be relatively high at that frequency. With outdoor measurements, the ULS would be no more than 1-2db down at 20Hz compared to 60Hz.
In our own demo room, where we don't have a deep bass suckout, THD @ 1ft from the ULS-15 was actually less than 5% @ 20Hz.
So the moral of the story is to use some caution when comparing results from one room to another or to outdoor measurements. In fact, even a small change in position within a given room will mean that data cannot be easily compared between one sub and another. For example, the ULS must be in a different location than the JL subs that Dr. Johnson measured, because there is not a prounced 45Hz suckout on the frequency response graph at 2m distance with the ULS.
P.S.: Even if not directly comparable due to potential differences in positioning, the THD measured from 25-50Hz @ 100db @ 1ft was even less on the ULS-15 compared to the F113 (note that 25-50Hz data is not shown in the review). Also, the frequency response is much more extended than the F113 too.
Pete - Hsu Research
Regarding your other question:
Somewhat shocked that there are only 5 pages on this ULS 15 sub (and not really that many on the HSU forums either)? Curious as to why that is?
Two reasons really, they are expensive,
therefore most ppl are hesitant to spend that much money on subwoofers. And 2 we are having so much fun with our subs that we don't post much.
I have probably been the most vocal on the merits of these subs. I used to do Ht installs and to me these are some of the best sounding accurate subs I've heard to date. You have probably read the reasons why I went with the HSu over the SubM, so I won't spend time here.
One last thing I would like to add is most of the customers who have bought these subs are not the type of ppl to post on any forum.
If I was you I would try the Dual Drive before selling the Ultra. Hsu does offers a 30 day trial and I would not want you to get the DD and then not be completely happy. You would be going from a ported to a sealed design and as you know they will sound different. Would that be an option? I have no doubt that if you do like them the Ultra would not be up for sale very long.
Hope this helped.