I use an EV Sb121a self-powered 12" sub (300w) that is the same compact size as the EV Sx100 speakers. It does a nice job filling out the sound and really gives punch to the bass and drum sounds. It weighs about 36 lbs. and is very easy to handle. I use this in connection with my Mackie 808s powered mixer and a pair of 300i 2-waqy speakers. It too makes for a great, compact, light set-up.
http://www.electrovoice.com/electrovoice/EVfiles.nsf/lookup/Sb121a/$File/SB121_534930.PDF

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Cass Pawlowski - Motown
PSR2000, SC88, Cakewalk