You did say you where a soundman didn't you ?

Not heard of watts equal v squared over r ?

eg an amp powered off 9v battery into an 8ohm spkr will give almost 10w
providing it can draw enough current and can drive rail to rail. Does your
outboard have a generator that can charge a car battery ? you could mount a
cheap car radio cd player that supports mp3's or even has an input for an
ipod which would be good for 20w/channel.

Pete