Any idea what is this?

Hi fellow amp builders!

My 6V6 Bassman/Plexi is exhibiting some strange behavior. Even when set to pristine clean, the bass notes have a distorted, buzzy resonance to them. As I go up the fretboard, the resonance becomes less noticeable.

On the scope everything looks ok except on the junction of the PI coupling caps and the power tube grid leak resistors. I thought it was a cap gone rogue but even after replacing the cap the problem persists. This funky waveform appears only in a narrow signal range.

Any ideas?

Here's the buzz recorded through the Palmer PDI-03:


And the waveform

 

Yes! I cannot remember by heart without opening it but I am pretty sure I put the 1.5K.

Why would they appear only on one side of the PI? And any ideas on how to solve them?

Funny thing is, on the scope they go away as soon as I reduce the master a bit (grid leaks are a dual gang 100K pot), but the buzz happens even when I set the master to 50%. Which makes me think that the buzz and oscillations actually might not be related.