Sauraen and Oikanys are working on the MIDIbox aSIDity project. This project will be the most powerful SID-based synthesizer in the world. The system will have 8 Commodore 64 SID sound chips (4 x 6581 and 4 x 8580) 160 modulators and many control options. At the moment they have completed the feedback system, build with an LM1973 (digital volume control) and an op-amp (3x gain). The controls are: Feedback, Cut-off, Resonance, Pulse width and Release time. You can watch the demonstration on YouTube.