Thanks but unfortunately that isn’t the case. Even with resonance and DCO1 & 2 volumes at 0, I can hear all waveforms when I press any key.
I started troubleshooting starting at the power supply first, and found an issue with the M5230L IC, and recapped the power supply (see topic https://forums.syntaur.com/t/jx-8p-power-issues/4229/23. The issue was still present so I decided to replace all capacitors on the main board, and the issue is still present.
Unfortunately this is the limit of my troubleshooting capabilities. It must be a bit of an unusual issue as I cannot find any information about similar issues online.