It only takes an extra 2" of wire to turn a bridgable 2-channel into a mono, so don't get turned away by a 2-channel. Just make sure it's bridgable if that's what you're planning to do.
Honestly, I've never heard a louder sub before for what little room it takes up. It's every bit as loud as 2x 12's and I still have most of my backseat. It's super-loud on the long rumbles and hits extremely crisp in the fast stuff. It also looks WAY cooler than pretty much any other sub too. I started with my amp gain up about halfway when I first installed the sub, and over time it just gets louder and louder as it breaks in. I've turned the gain clear down to about 1/4 on the amp.
Here's the first setup I had. Dual Rockford 12's, the 900W Anniversary amp, and Godzilla independent ported boxes.
Then those got stolen right out of a freakin' parking garage. I barely got them in taking the front seats out, so I haven't a clue how they managed to get the things out in one piece. Thankfully one amp channel was shot, the head had quit spitting CD's, and one sub cone was damaged. They really didn't get much in the steal. Replaced them with this Kicker 10" L7 Solobaric and Boss GT580 1000W amp in a sealed enclosure. Love it. And, obviously, I can put stuff in the backseat again.
Excuse the Taco Bell and random tools!