Imagining basically a mixer circuit where outputs of amps can be routed back to inputs to create feedback. Possibly multiple. Include a mcu with ADC to detect levels at certain points, then control the feedback via digital pots with the mcu. With detecting level + ability to control feedback, I'm imagining a sustained tone could be created that would be kind of dynamic. Depends a lot on how the amp circuits are made. Would probably include some filtering in the analog realm too. DAC possibly as well, to do some DSP. But the main focus would be controlling feedback by detecting levels and dynamically adjusting gain. Could have an input too.