I don't know where you got that a 1-12 can't be used for bass. MANY good amp companies, such as Aguilar, SWR, and Peavey, have 1-12 speakers. Ampeg has a BA-112 - 50 watt bass combo through a single 12" speaker. So blowing the speaker because it can't handle the lows is out of the question. A 12" can actually produce lows better than any 10" can - and almost every bass player plays through 10" speakers. However, I think you may be right about completely overloading the power section. You can't plug the PA head into that combo cuz it is both a pre- and power amp. The two plugs you were talking about, the power amp in and pre amp out, aren't designed to work with 2 sets of something (ie. if you plug the head into the pre-out, there are two pre amps and vice versa). The only way I can think of what might work is if your PA head has a power amp in and pre amp out. Then you could run a line from the pre-out of your PA head to the power amp in of your combo. This is still REALLY risky though, and I don't think that would even work, but you could give it a shot. I'd say your best bet is do what jokerjnky said and sell both the combo and the PA head and buy a real amp.