Plugging in headphones - effect on volume tuning

Before I call Moog Music for tech support, I thought I'd see if anyone else has had this problem with their Ethervox MIDI Theremin and whether there's an easy fix.

When I play without headphones plugged in, I can control the volume just fine. But it seems that adding headphones has just enough effect on the volume circuit that you can't silence the volume anymore (except by touching the volume antenna).

There don't seem to be any easily accessible controls for re-tuning the volume circuit. Any ideas?

Several Ethervox players on levnet have said their instrument got out of tune over time and needed adjustment. I suspect it's just quite sensitive, and could be reacting to minor variations in the Earth's magnetic field (it has them) or ionization in the atmosphere. (The latter changes in relation to the 22 year sunspot cycle). Just call Moog and ask, they can probably tell you pretty fast how to make the adjustment... although sadly it's going to take some time to actually do it. (Turn off and unplug instrument, open case, make tiny adjustment, close case, plug in and turn on, try it, repeat until it's right.)