Does anyone have any ideas on how to keep the levels down so that all the instruments can be heard?

The only way would be to take responsibility for the volume control away from the electric musicians. Set the levels at the start of the session and stick some gaffa tape over the volume knobs. Anyone found tweaking the volume knob in the upwards direction gets a whack on the knuckles for the first offence and a power cut for the second.

It's only human nature to want to hear yourself and be heard better, and/or to 'improve' the overall sound by having your instrument that bit more prominent; and once one person does that, the game is on and it's a race to the top.

The acoustic musicians will have to put more physical effort into playing to get more volume, and will have a finite limit imposed by their instrument on how much louder they can get - the electric musicians also have a finite limit of course, but more volume is much easier to achieve by twiddling a knob, and the upper level is only reached when their volume knob won't twist clockwise any higher.

There are, of course, many musicians and audience members who directly equate sound pressure levels and musical quality, but that's a whole different kettle of fish ...