None of those are batter no joke.
RE audio MT series are pretty sick for SPL, that's for sure but is overpriced.
http://www.soundsolutionsaudio.com/store/products/18%22-Death-Row-Series.html this is another good one I would suggest you take a look at it, this woofer is not that loud but it'll hit almost as same as RE audio SXX series, check this one out for sure. http://www.soundsolutionsaudio.com/store/products/15%22-Death-Penalty-Series.html a lot of louder. 21 Inch warden series from Incriminator Audio the Kicker Solo X 18 inch can't even beat it in the competition.
single http://www.sonicelectronix.com/item_18537_Orion-2010-HCCA102-HCCA10.2-HCCA-10.2.html only 10 inch woofer will blow away both of SXX and PPI PC away, the PPI woofer is ratted at 1600 watts of power, Woofer Composition Paper for the PPI woofer, the HCCA is Polypropylene for the woofer composition should be batter than the PPi ones, if you don't believe call them up they'll tell you the same thing what I am telling you now.
http://www.ascendantaudio.com/mayhem.php another good woofer I would suggest you to check it out, 15 inch woofer is like 2500 watts of RMS power. nice looking too.
Fi audio BTL N2 12 inch is another good one, handles 2000 watts RMS and 4000 watt Peak one of my friend only have one of those babe 15 inch running on Alpine M2000 hell yea blow the crap out of his car, he is now thinking about change it to 2 12 inch BTL's running on 2 Alpine M2000's but it will be almost twice as loud as what he have now.
another woofer I am in love with is the Clif Design Aftermath series 12 inch woofer running at 1800 watts RMS 3600 watts of peak power. Not as loud as 12 inch Fi audio BTL or 12 inch RE audio MT series, but dose hit a clean bass all that. Blows both 18 inch PPI and RE audio SXX series out of water for sure.