Just to followup on this: I took my tuner into Empeg, and reproduced the problem there. Hugo seemed as baffled as me (more so, since he's supposed to know this stuff ), and solved my problem by replacing the tuner module. I left the suspect adaptor with him (if I'd had my wits about me, I'd have haggled for an extra remote in exchange!), to look at it with a 'scope to see if it's sending anything suspect.

I made my own replacement adaptor - see the Installations board.