tonight:
- modded GBC with "pro sound" mod: basically, tapping the audio at the volume knob to bypass the headphone/speaker amp circuitry
- this improved the sound & bass response a lot but noticed a lot of low end and sub-bass still missing
- checked GBC schematics, there's a 300Hz high-pass on the mixer output, before the amp
in theory adding extra capacitors in parallel with the existing ones, or replacing them or the resistors, could fix this. but that is a project for another time.