I was at Guitar Center about two months ago and a sales guy there was demo'ing and Markbass head of some sort. There was something about the input that could power an active pre-amp without the need for a battery.
Just the other day, I came across this from EMG in both a 9v and 18v flavor. http://www.emgpickups.com/products/index/379/292/4
Does anyone know how well these external power supplies work and if there are any other brands available?