My 20-39 has made me very happy - I didn't have a need to keep hunting for anything else! I haven't heard any Hsu, (or the Axioms for that matter) but even the SVS guys will tell you you can't go wrong there either.
The SVS is tuned to, for instance, 20hz in the case of the 20-39. It has three ports. By plugging one port, the tuning drops to 16hz, and by plugging two, down to 12.
(The 16-46 is already tuned to 16, with no plugs.)
The result is, necessarily, a drop in output across the board, something like 3db with each plug.
Having said that - my 20-39PC+ has no plugs in it, and can be felt shaking my floor down to 10hz (this is using a demo disc with tracks starting at 6hz). I can't really "hear" it til 14 or 16. And the subsonic filter prevents it from really thumping until you reach the tuning point.
