I have the 1500D And its sex. Bass is awesome, really fills up the room, and with my 2x12" PAs, can handle at least 100 people outside, and 150-200 inside.
So for the passive 18" version, i would assume it would be even better, and could easily do a few hundred people in a good venue.
In regards to sparkbro’s response, the 1800D powered sub blows away the 1500D (although the 1500D is still an awesome compact powered sub if you ask me), the Pro line is supposed to be a big step up still, throw an EP 4000 on a pair of em and I bet you could rock some parties.
Don’t get me wrong, the 1500D is a great box, the amount of bass coming out of a box that small still blows me mind (until you sit it next to a SRX718S that is, but we’re talking a good $1k price difference, plus an extra power amp). I brought my S2 into work one day when it was slow and plugged into a small rig (2xB212XL and 2xB1500D), people walking by were coming in to check out the noise.