I used a Hog Tunes amp, cost is around $600.00, it's 160 watts and comes with 4 speakers. It's made to power the front and rear speakers but I made a slight modification and have it powering the speakers in the lowers instead of the rear.

 Overall I'm very happy with the set-up but there is not much volume from the lowers on the highway.

 The whole system took me the better part of a day to install, I'm a retired industrial electrician with a history in the instrument trade so it wasn't all that hard.

 Right now there are no speakers in the rear pods, sometime this winter I'll get a 2 new Hog Tunes speakers and install them and a set of plugs so I can switch between them and the lowers when I remove the tour pac.
 Speakers in the lowers look great and do put out some sound.