I mixed on four of the Mackie 18" powered subs in a club two weeks ago. They got down to 30Hz solid and tapered off below that. Nice and full up front, but by the back of the club, maybe 75 feet from the stage, the bass was still there but not really shakin. The guy that owned em said they had a little more to give.
It kind of depends on what you want, these boxes get low enough to shake your guts, down there in the sub-bass. If you want to feel the kick in your chest, look for boxes that center in around 80Hz.
Meyer will have something to suit your taste, although maybe not your budget. Yorkville, believe it or not, has some powered subs that I've felt shake pretty well.
Are you mixing live, or is it strictly for playback? I've found 18"s to be good for both, and high-powered 15"s have treated me well in dj situations. I haven't had much experience with powered gear aside from Meyer.
Have you tried the JBL Eon subs? (might be an obvious question)
We use the Cerwin Vega T-36/750 in dance clubs and party applications, but they do rock really hard. I would suggest also using a driverack or other dsp, and making sure you have good amps to drive them, like Crest. I'm not a big fan of powered speakers, but the JBL M-pro line isn't bad.