Regarding performance, The Hsu subs don't have any real flaws relative to other brands. But, like I said, between the manufacturers discussed here, none of them have any real performance weaknesses. The trade-offs are more in other areas like pricing, feature sets, warranties, shipping costs, etc.
Yes, the Hsu subs have very good distortion characteristics. Not quite as good as the Monolith THX subs, but those are engineered to have extremely low distortion in order to achieve THX certification. The THX subs from Monoprice and Outlaw Audio are some of the lowest distortion subs that are available. The Hsu subs do have distortion so low that it would be inaudible at almost all drive levels. In other words, you would have to crank the Hsu subs to maximum drive levels for distortion to be heard at all, and you would probably never do that. If you want to see the data, look through
Audioholics subwoofer reviews for the subs you are interested in comparing. For Hsu, there is the VTF-1 mk3, VTF-2 mk5, ULS-15 mk2. I also have extensive measurements for the VTF15h mk2, ULS-15 mk1, and VTF-3 mk3 that are not published.