When I think "old school tone" I certainly don't think GK>Bag End, I think the Ampeg would be a much better choice for the tone you're going for. That amp can get very close to a B15 (much closer than the current "Portaflex" line, IMO) and a P-bass with flats trough a B15 is one of the best bass tones ever. A B100r is pretty darned easy to move around. Plus, you only have one unit to carry and the only things you'll need to plug in is the power and your bass - it doesn't get much easier than that.