I emailed the guy a few weeks ago on this same sub - he's selling on ebay for $300, so you probably can negotiate price if you are interested.
The amp is Rythmik, and I asked him what it brings to the table. It's not a servo model amp (and even if so, it's not a servo subwoofer), so it sounds like it's doing the same thing any BASH amp would. Sealed builds should be pretty idiot proof as long as the box isn't too large and it's braced / dampened properly. Do the knock test (knock on the sub at various points and see if it's a "echoey" sound or a solid sound) before you listen to it demoing anything. Maybe you could also remove the amp / woofer for the sake of looking inside and verifying before exchanging money. The sub probably sounds good, but as abs noted, frequency extension may not be low enough for satisfying home theater.
I get a lot more mileage out of buying subs and amps w/o enclosures on craigslist and then building what I need. I'm rocking 2 DIY builds now, and definitely saved money with that approach.
Originally Posted by rhenschel
If you are using a good amp and a good driver, is that basically all you need? Or is it important on how the box is built, etc.?
3 things are needed for a good subwoofer. A good amp, a good driver, and a good enclosure (matching may be better term than good here). The enclosure determines how low your sub will play, and poor enclosures or poorly designed enclosures can have ringing, distortion, or other generally bad sound artifacts. There are a lot of subwoofer building primers if you want to look into it, but the general idea is to match all 3 to a happy place. A good enclosure and a good amp would go to waste with a crappy sub driver, and a good amp and sub would be wasted with an enclosure that was poor or not right for it.
Sealed is generally good, and it is a favorite of many in the DIY forum, but it's usually not the best bang for your buck when shooting for loud and low playing subwoofers. Ported usually gets you more for less, but careful design and modelling become a lot more important then as ported can sound really good or really bad depending on design and how well the build is done.