Another option for on-board amplification is the Boosteroo. They have a variety of portable amps including a 12 volt version which I have hard wired into the bike. Doubles the available volume of your mp3 player.
Boostaroo web page

Cheers, Richard
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09 America, Staintune Pipes, K&N, Breathe, Hagon Nitros, AI & O2 removed, tune 20184 (modified), MTX-L a/f gauge