Korg ESX audio issues

Hey all, just brought a second-hand Korg ESX-1 sampler which is in great condition but it has one issue.

Audio on the left channel sometimes either gets distorted or disappears from either headphones or the main left output.
I initially thought it may be the vac tubes and swapped them around to see if the problem then went to the right side but it didn’t.

Thought it may be the volume pot as the issue would sometimes fix itself if I turned the knob however I can also seem to fix by turning up the tube volumes too.

Any ideas before I crack it open to check the solder joints etc??

Thank you in advance.