I'm interested in very mixed sessions, where acoustic and electric / electronic instruments can be mixed. On the few occasions where I've seen it done though the electric instruments (guitars mainly) tend to gradually push up the volume to the extent that the acoustics can't be heard.

I don't know if it's specifically an electric guitar problem, given that most guitar amps are potentially very loud (even the low-power ones) and also that they often sound better at higher volumes, particularly for over-driven sounds.

Does anyone have any ideas on how to keep the levels down so that all the instruments can be heard? Or experience of successful sessions of this kind?