Hey there just wanted to let you know, if you have these issues, check for faulty OPA2134PA. In my case 2 of them were not working properly.
I tried to swap them out with each other, but could not really find out which was the one.
In different positions you get lower output volume, or exaggerated HF harmonics or no signal at all with all LEDs on.
I found out that these OPA2134PAs should show about 12-14 kOhm when measuring at pin 4 (V-) and 5 (V+).
One of mine showed 25kOhm and the other one infinite resistance. I sorted them out and tried to put 2 in from a second kit I ordered. After that it was working as expected.
Regards