In all these debates about subs, on here, Avs etc, people always talk about listed specs, max SPL and differences of a few dB. I think in real life maybe 1% of subs will ever reach that limit, so those differences are moot and will never be heard.
The fact is all the ID brands being discussed like Hsu, Svs, Rhythmik etc are going to be indistinguishable in a blind test and indeed in the many GtG's I've read, that has been the case.
So buy based on price, size, extras (like warranty, free returns) etc, factors which get ignored. If you are one of the few who will actually play a sub near its envelope (and even with reference level playback thats unlikely in most rooms) then by all means the specs matter more.