IMO, the TREO would blow the XXX away in SPL...hands down. these subs are ridiculous.
i have heard 2 SSX 12's in a hatchback(300zx)...152dbs.
and 2 SSX 15's in a Tahoe, not sure what he hits, but he pulled up behind me one day while i was in the line at Taco Bell, and i had my system up quite a bit, with my windows up, and all i could hear was his.
Now on the other hand, the XXX would kill the SSX in SQ IMO. i find that the SSX has decent SQ at lower to middle volume, but they aren't designed for SQ, so once those subs start movin, it's just balls out Bass.
the XXX was designed solely for SQ, that's why it has the Xbl2
motor technology, but it was tweaked a little more to get more SQ, and made the sub very inefficient, thus needing more watts to achieve the same output.
i have heard a 15" XXX in a Jeep Cherokee, it was extremely loud, and sounded Excellent, but it doesn't quite compare to the SSX. but these subs were designed for different purposes.
the XXX gets so loud b/c of its power handling and excursion capabilites but it is still a SQ sub.
i also wouldn't give the XXX almost double its recommended RMS rating.
a lot of people favor RE, b/c they are very well built subs, they sound great, get loud, and are cheap price-wise for what you get. they also handle a little over their RMS rating, but like i said earlier, almost double is pushing it.
i would say the XXX would win all around for a daily driver, it sounds great, they get very loud, and cost considerably less than the Treo's.
not only that, but you can buy them from their website, unlike Treo, you have to buy them from a dealer, well not everybody lives next to a dealer, so it is more convenient to just buy straight from their website(cheaper too)
hope this helps a little.