Help! Studio 22 plus signal dies and distorted

Hello Boogie Board.

I've had this MesaBoogie Studio .22 plus (Graphic EQ, Spring Reverb, the work) in my possession for a number of months, and Id like to get it situated and out of my house.

The issue seems to be entirely circuit board related. I've tested the speaker (it was a replacement and I just wanted to be sure) and its all good.

I replaced the tubes with a fresh set from Sovtek just today and the problem persists.

Ive gone through the board with a multimeter and measured the components in the preamp and power amp circuit and nothing seems to be out of the ordinary in terms of measured values.

Ive tried the patch cable across the effects loop and it doesnt make a difference. Tried switching the EQ switch through everu stage, checked the fuse, tried the ground lift stages, tried just about everything that I could switch and swap.

Im honestly at wit's end here. Id rather not throw any more money at the problem

I'll link a YouTube video in a bit here to show you exactly what's going on

https://youtu.be/7ZUP4465dlE

Is the loop series or parallel in these amps? I'd like to know what happens if you run the preamp into another amp's power section and another amp's preamp into this amp's power section.

Also, are the tubes heaters staying lit when the amp fades out? Or are the power tubes red plating? Try playing the amp in the dark.

I was able to pull a schematic from google fairly easily. I believe the effects loop is in series.
Considering the power amp and preamp are on the same board (there's only one pcb in the entire housing, anyway), I didnt consider it let alone believed it was possible.

I just checked in my schematics directory and it is in series.