Power supply is a good place to start. Check that all the voltages are correct and that nothing's sagging under load. Turn all those trim pots back to where they were (I hope you followed good practice and marked their original positions!) as that isn't the way to increase the volume as you've found.
With an oldie like this, you're very likely looking at changing all the electrolytic caps in the power supply and possibly elsewhere. Hazardous voltages involved here, so read the sticky on procedures first, unless you're already experienced in high voltage repairs.