Thanks for your help everyone. I’m going with a new Electro Harmonix 6CG7EH as that’s what was in it. I gather this should provide the intended tone and “voice” as you mentioned John and Spritzer.

  It has Electro Harmonix 6CG7 tubes and switching them over made the opposite channel work OK so it would appear to be a tube failure. Not sure of the root cause though.   Have read the Tube Change and Biasing thread and consider this is something I’d be quite capable of doing. Is it best...

Just fired my STAX SRM-006tS up this morning -- after a week or so of it lying dormant -- to discover it playing one channel of audio only. On inspection, it appears as though only one of the two tubes is glowing. Trying input 1/2/3 makes no difference as does either earspeaker output socket...