So, I got the selector switch to work. The only thing is I'm getting some bleeding from other inputs if two sources are active, but that is ok for now.
I didn't mean to hijack this thread, but if there is anything I can contribute to this thread related to bent modules and headphones is that it should work. After fixing my wiring issues, with the help of armaegis, I got both pairs of headphones: the Beyerdynamic dt880 and the Fostex trp50mkiii, to work. I realized why the Fostex sounded so muffled and inefficient compared to the Beyerdynamic, which were these custom earpads I put on distanced my ears too far from the drivers which caused them to sound less loud and clear and I think the padding caused some muffledness. After swapping the pads for the originals they played as loud as the Beyers.
Am I not sure about sensitivity and impedance with autoformers now? The Fostex are 92db at 50ohms, but they play as loud as the Beyerdynamics dt880 that are 96db at 600ohms which are, on paper, a better match. This somewhat mimics John Chapman's experience. Is impedance irrelevant with autoformers and all you needs is a baseline sensitivity? I'll post more findings soon.
In summary, autoformers work with headphones. You need 2 volts and a source under 100 ohms for sure, but the headphone match is still a mystery.