New data point is that I got a TRS–>TS adapter and tried that with the M-Audio, and the Eventide seems to only intermittently recognize it. Setting the range control on the side of the pedal to one of the extremes seemed to work… until it didn’t. As I say, I have an Ernie Ball volume pedal that works more or less, using a TS jack, but it doesn’t seem to calibrate, and the whole “throw” from 0 to 100% only uses about 1/3 of the pedal range, which is awkward. Do they make a dedicated expression pedal any more? I haven’t been able to find one. I’ve looked at others and they all seem to have TRS connectors. The UM has a link to recommended pedals page, but it’s a 404.
I’m starting to wonder if it’s a problem with the Eventide jack itself, but I don’t know how to definitively diagnose it, not being an electronics tech. Is there a list of Eventide-authorized repair shops somewhere?