OK folks, just got off the phone with Richard at AKG. His take on the VR tube thing is that it *could* be possible, but the tube provides ground to the circuit.
So find an alternate grounding source and maybe it could work? (these are my conclusions, not his).
His take on it was that the demand wasn't there, so there wasn't any reason to do it. He definitely did not come out and say "it can be done", but once I explained the thought process to him he did concede that it might be possible.
So there you go.