I've heard great things about HSU, but have never heard one. Would the HSU HST-2 be a significant upgrade over my Velodyne CT120 or would it even be an upgrade.
I'd stick with your Velodyne if the HSU is still a single 12" driver sub. I just don't think your going to get a drastic difference between the two. It probably would be a small upgrade though.
You can still find decent deals on CHT-15's... Or maybe even a good deal on their newer mid-fi line stuff like the DLS-5000R
I'm a little biased, I'm sort of down on single 12" drivers in general in subs these days, but I demand a lot out of my subs too.