ok...here's what I got and what I recommend.
WoofersEtc.com - 3XL-151D - Powerbass 15" Dual 1-Ohm Subwoofer
The last 5 years, I have been running a pair of these on a 2200 watt Audioque @ 1ohm...never had a problem and I would not trade them for anything I have seen around here..they get stupid loud. I do not compete, so I have never tried to see how far they go. I got them locally for a deal that was hard to pass....same for these...
WoofersEtc.com - 2XL-154D - Powerbass 15" Dual 4-Ohm Subwoofer
I have been running 2 of the 2XL15D's in my wife's ride on a HiFonics Brutus BXI2005D @ 1ohm for the last 6 yrs, again, no problems and for the money, would not trade them for anything either... and she is alot harder on them than I am on the 3XL's.
Supposedly there are no Authorized online retailers but you can google them..I bought mine new local for less than half of what they are at woofers etc. I bought almost everything in both systems (mids/highs, front amps and the brutus) from woofers etc with prompt service.
Powerbass has an excellent CS, I used them for recommending and tuning both my boxes. I do not mind recommending them or their products to anyone looking for a loud and clean system.