ACI only sells direct, just like HSU. I have never heard the HSU, so I cannot directly comment, but I own a Titan II LE and it is one spectacular sub. For music, this baby cannot be beat and I'm sure it has a lower Q rating (because of it's sealed enclosure) than the HSU, which I believe to be ported. However, because of the different design characteristics, I'm sure the HSU will be able to put out higher SPL's for movies, if that's your cup of tea. In my trials, the ACI has PLENTY of output, so depending on your room, you may not need more. This sub revolutionized my system. I never realized what I was missing with my small Mirage. The bass is tight, quick and powerful. It just disappears with the main speakers as well after integration and calibration. It's the best piece of gear I own in my system next to my Anthem AVM-20.
Of course, I would give a large advantage to the ACI in terms of looks as the ACI looks like furniture grade quality woodwook and works perfectly as a small end table. The fit and finish is spectacular.
Both companies have a great trial period. You should order both and return the one you don't want. You will be out on shipping a bit, but at least you will put all doubt to rest.
agree with evan- completely different marketing segments. I'd bet no one has compared the two side by side. simply put- music vs HT. why dont you order the two and find out for yourself and report to the rest of us?
"I'm sure it has a lower Q rating (because of it's sealed enclosure) than the HSU, which I believe to be ported."
While this generally tends to be true when comparing sealed to ported subs, it is not always. There are certainly some hi-Q sealed subs around. And likewise there are some excellent low-Q ported subs if you know where to look (SVS or the Adire alignments for the Shiva and Tempest).
I think you were alluding to the fact that group delay of a sealed sub is lower than ported. This is certainly true for the vast majority of alignments. But a well designed, low-tuned ported sub will have a group delay peak that occurs in the subsonic range so that the audible group delay is minimal. In this way you can achieve high accuracy AND output - everybody's cup of tea.
I haven't heard either sub so I can't help much there. Sorry if I went too far off-topic.