Whooo, it works! Behold the monstrosity...
(not shown: the Fuze providing music)

- Sansa Fuze for music
- s22 providing power (+/-18V) for the ABG circuit and the three a20 boards
- the ABG circuit feeds OL, OR, OG each into its own a20 board
- connect output from each a20 board to the headphone
- no resistors on the current ABG circuit, using INA134/137 which are differential receiver chips that have built in resistors
At the moment I've only tested it with cheap airline headphones so I don't really know how good it sounds. The a20 boards are extremely susceptible to temperature variations and produce voltage offset due to that, so I'm going to rig up a temporary cardboard box to protect it from drafts (like from my arm movement or breathing) before using my good headphones.