You should check out the Awesome Bose P.A. Systems!
I absolutely love mine as does everyone else i've encountered with one.
I can easily fill out a 2 to 300 capacity room with top quality sound using only Logic backing tracks, my Korg M3 and my voice!
Its expensive mind but definitely worth every penny!!
Check it out here: http://www.bose.com/controller?url=/shop_online/speakers/portable_amplific ation_systems/index.jsp
And check out their brilliant forum here: http://bose.infopop.cc/6/ubb.x