Several British magazines have reviewed it favorably against the similar REL Q150E (which is rated at 20 Hz I believe - though at about -6dB).
My only concern would be room size. My room is 27' x 14' with 8' ceilings and several large doorways. I contacted the owner of the company, Phill Mason, and he said the Pro 50 would suffice in my room, but if I was really playing things at high levels and lots of movies and wanted greater impact he would go with the Pro100.
I am looking for a sub that seems to be similar to what you're looking for. I would like a musical sub (and for occasional movies - but my thought is that if it's great for music, it couldn't be bad for HT), that doesn't take up a lot of space, and is around the $500 price point.
To this point I'm looking at the MJ Pro 50, M&K V75 and the REL Q150E - which is getting a little past my budget. The M&K's and REL's are readily available here. The MJ subs are not in the US yet, but I can still get the Pro 50 to my door for a little over $500.
Let me know how your search goes.
Chris