Many thanks for suggestions and links to Ebay. The HD 65 is billed as being for TV, so I wonder if it's all that brilliant for music. Also, I just discovered Sennheiser’s HZR62 stereo volume control, apparently with a separate slider volume control for each channel.
As you’ll have gathered, I’m no expert. With loudspeakers, I’ve never heard a strong stereo effect (even with hearing aid on one side), so I've not paid much attention to balance. But I always thought balance was different from volume: i.e. shifting the apparent microphone position without changing the volume for the listener. If the use of balance really does shift volume from one side to the other, then maybe I’m worrying unduly!
Plan is to try regular headphones and, if not satisfactory, add an HZR62. Thanks to all.