Although very new to the market and yet untested, I vote for the Hsu Research ULS 15 for two reasons:
1. I owned a Hsu Research VTF-3 HO Turbo, and it was very musical although it was ported. Sounded great
2. The ULS 15 is a sealed sub, and many agree that sealed subs are more "punchy" or "tight". I expect nothing less than the best from this design.
Like most Internet Direct companies, Hsu Researrch gives a 30 day trail on all products, so no worries
http://www.hsuresearch.com/products/uls15.html