AA864 Vibrolux Reverb noise issues

[argonaut]

I've been pulling out my hair trying to solve a noise issue with a '65 Vibrolux. I was hoping I could get some assistance. The amp making an undisireable noise when its volume is turned above 3 or so when playing through either channel. It sounds like a low 'farty' distorted noise riding atop the notes. It's most noiticeable when the amp is up in volume and when playing between the F and A notes on the low E string.

I suspected a problem with the PI as the noise is present in both channels. I quickly confirmed this with an audio probe. As both channels have a nice clean signal until it reaches the PI grid that connects to the NFB side. There it becomes all squashed and nasty. I did find a few out of spec resistors and replaced them and also tested all coupling caps. Not to mention trying all sorts of tubes, cleaning sockets, checking grounds, and the usual. I replaced all the electrlytics 5 years ago. So, I'm confessing my ignorance........I'm stumped. What else should I look for?

Here's some additional info that relates to the PI:

All measurements were taken with a VTVM

323V B+ feeding PI (12AT7)

1- 190V
2- 68V
3- 72V

6- 167V
7- 69V
8- 72V

I ran a 1K test signal into the amp and took a pic of the PI grids. The pic labeled PI-neg is pin7 (see attached). Also attached is an AA964 schem that, at a quick glance, looks just like my 864. Thanks in advance for any help!