The UM-18's aren't designed for parties, they are designed for generating lots of low frequency bass <35hz.
(I'm sure it will be a lot louder than whatever you had prior though...)
In that price range, this driver would be better suited for parties:
Dance music is all in the 40-200hz range, where high-excursion drivers won't help you.
If you dance to this, the UM-18's will help you generate this 30hz bass...
If you dance to mid-bass stuff like this, then a number of Eminence, B&C or PA-460's would suit you better.
For dance music you want a woofer that has an impedance-peak in the 50-100hz range when box modelled, and a driver with the highest sensitivity and power-handling combination that you can find.
Then it will get loud and stay loud, without overheating or sounding weak.
The Eminence Sigma Pro 18a is about 8 times louder than the UM-18's at this task. You'd probably only need four of them.
For dual-purpose, you'd want to stay with the UM-18's though.
Unless you party more often than you watch movies... that is.