I'm always rather wary of things that are too clever, because if they go wrong they go horribly wrong. Series connections mean a cable issue has the amp running into open circuit, which is not good. Its not usually that great an idea to run odd speakers either, although you can get away with all sorts of misdemeanors with a guitar amp that would make a bass rig very sick. I'm with Simon though, I think if you need more volume than a 1x12 it would be better just to use an external cab, the saving by keeping the internal speaker in the loop isn't worth the complication.