Hello bob540,
I've never owned a HSU sub but wouldn't hesitate to purchase them based on their very good reviews, reasonable pricing and generous return terms.
However, I would advise buying a pair rather than just a single sub. Based on my experience, a pair of subs will perform and sound about twice as good as a single sub. Bass is cumulative and having 2 will provide a more accurate, realistic and natural presentation of the bass impact and dynamics recorded on the source content with neither sub operating anywhere near its limit.
Having 2 subs running in mono mode, properly positioned in your room and in relation to your listening seat, will also provide the normal multiple subs in a room benefits of the bass being smoother, faster, more detailed and better integrated with the main speakers than a single sub is capable of. The soundstage normally sounds wider, deeper as well as more realistic, open and natural.
Besides choosing a pair of HSU subs that match your physical size and budget requirements, you need to make sure the subs have separate controls on each for volume, cutoff frequency and continuously variable phase, I believe most if not all HSU sub models have these. If your budget is tight, I recommend that utilizing 2 smaller and less expensive subs will perform better than a single larger and more expensive sub in virtually any room and system by a wide margin.
Useful skills to learn are how to optimally position and configure 2 subs in a room. I suggest googling the 'crawl method' for positioning and setting the volume and cutoff frequency controls on each sub as low as possible with the bass still sounding good to you (powerful, dynamic, detailed and natural). Remember, the goal is not to constantly hear and feel the contributions of the bass from the subs but for them only to supplement the deep bass when the content calls for it.
Best wishes,
Tim
