Just phoned for a progress report. They can't find anything wrong with the headphone socket. This is puzzling as I've tried it on two sets of phones and the results are identical. The phones work fine on the Arcam and the PC.
They're going to re-cable the headphones, to be on the safe side, and they'll ship it back to me early next week.
They're going to re-cable the headphones, to be on the safe side, and they'll ship it back to me early next week.