Finally getting a chance to get down in the studio again to do some mixing and with the chance of cutting more tracks looming large in my immediate future, I figured maybe it was an opportune moment to fix the wiring.
Step one was to download the manual for the headphone amp so that I could look at the connectors when not lying flat on my back. I determined that the Main In connectors were L/R balanced tip/ring/sleeve and checked the Big Knob to make sure that the cables matched that configuration, which they happily did.
Ok, on my back under the console now. The left channel input is plugged into Main In L, the right channel input is plugged into Main Out L.
Well, that would explain a lot.
The problem is that the headphone amp is not very deep in the rack, while the units above and below it are very deep. So deep, in fact, that I could not manage to snake my big fat hand in the gap to plug the single cable that needed to be moved back in.
A solution existed. So I unplugged everything that was connected to the back of the headphone amp, got up, yanked it out of the rack, and moved the really deep unit that had been above it down one slot. Then I dropped it back in and went to rewire it. This lasted until I decided it was still annoyingly difficult to reach and I called daisy_knotwise in to help.
I pulled the unit back out of the rack while Gretchen got down underneath the console and handed cables up to me to be plugged back in through the gap in the rack. Once the five cables were reconnected, I dropped the unit back in the rack, plugged in the headphones, and there was much rejoicing.
Or, at least, there was signal in both the left and right earpiece, which I will take as a victory. :)