Pre3 has lost its right hand channel

[NickW]

Hi folks,
I need some help! I came to switch on my Pre3 last night only to find it has lost its right hand channel. It sounds like it is completely dead, except that every now and then there is a very low-level and dull-sounding "plop" sound from the right speaker. Then silence again for at least another 15 seconds or more.
I'm failing to find what is wrong and so am looking for ideas. So far, I've tried:
1. Swapping over the valves and changing both valves. No change.
2. Swapping input leads to another set of input connectors. No change.
3. Swapping over the output leads. The problem moves to the left hand channel. So it is definitely something inside the Pre3 connected with the right channel.
4. I measured all the heater voltages. All within spec at 6.5V sitting on top of ~ 39V.
5. I measured both the cathode voltages. ~ 4.5V (pin 8) and ~ 9V (pin 3) as per spec.

I'm stuck as to what to try next? Any thoughts anyone?

Thanks, NickW