I was pondering on using a power pack slipped into a pocket for powering an active bass, instead of using batteries.   So the idea is to convert the battery connector to a Boss style 9v female, then a Boss type 9v male to USB, and then to the power pack.   Would this work or is there some compatibility issues with mA's or voltage or such stuff ?   1:  The 9v battery / 2.1mm female converter             2: Boss type 2.1mm  5v