I would get a single powered sub 15" for 100 guests 18" for 200 guests. I think the powered subs that have enough power to drive a matching passive sub are cool. You can lug the 2nd for big gigs and they are designed to work together.
That would be the best bang for the buck plus easy setup. I've used the JBL MPro's you won't be sorry.
http://www.jblpro.com/pub/mi/MP418SP.pdf
http://www.jblpro.com/pub/mi/MP418S.pdf
Normaly you want the sub to have twice the power of your fullrange speakers.
Forget subs with built in passive crossovers. They waste alot of power.
[ April 09, 2003, 08:21 PM: Message edited by: KJDJ ]