I  would try changing R76 to a lower value.
(coming off of the OD Lvl wiper.)

There are a bevy of class d amps available on the net.

Small & simple.

Pick one that utilizes the +40/ -40 Vdc supply that you have.

Why are you going down this road?

See if this is close:

"Well fellas, as I practiced putting filter caps In a second Peavey amp head I bought to tinker"

Which model amp are you now working on?

That decaying distorted sounds could be the amp itself (crossover distortion) or the speaker.
Things that you can try:
Another amp into that speaker.
The Peavey amp into a different speaker.

It's a 12 watt amp into 8 ohms.

You're really going to play a bass guitar through that?

Into 4 ohms, the amp will be expected to supply 24 watts into the load.
IF you play it all the way up, it will fail.

" I'm reading up on how to use it (the volt meter) to check for ripple voltage. "


Easy. Peasy. 8|

Set your meter to read Volts ac.

https://www.ssguitar.com/index.php?action=dlattach;topic=3606.0;attach=7940

I would give it a go.

If it does turn out to be the TDA2007A, I have had real good luck with this seller (they are located in England).
https://www.littlediode.com/components/TDA2007A.html?gclid=EAIaIQobChMIjoihq9rB3gIVxSaGCh0rCAsiEAkYCiABEgK7WvD_BwE

Very odd indeed.

Is one of the plugs TRS?

Thanks for the resolution.

Click on the link: https://www.ssguitar.com/index.php?action=dlattach;topic=3606.0;attach=7940

Here is the schematic set. (TGM30_Page 7)