Ok.
So you measured the + & - 25Vdc supply but what is on the opamp supplies?
Pin 4 & Pin 8.
What is the actual output of the LM1875 (Volts AC) with the speaker disconnected?
What is the signal amplitude at the input pin ? (1 Vac would be nice)
With a + - 25 Vdc supply the IC should be capable of sourcing 20 watts into a load.
(that would equate to a 12.5 Vac RMS output signal into an 8 ohm load)

" possibly even a schematic?"

This may be what you need.
https://music-electronics-forum.com/forum/schematic-requests/43021-baldwin-c2-schematic-needed?t=41936

Here is the Cross Ref sheet.

+12Vdc comes from Page 3 Qc7.
It must pass through Relay 1 (page 6) normally closed contact to get to U2, the 7805 input.

My hat is off to you.
So the feedback path was the issue.

Not too sure removing parts is the way forward on this.
That 200 mv signal is coming back on the feedback path.

Sounds like a mighty big challenge.
The front panel in itself is going to be a challenge on a tag board.
Check out this link as they have PCB file info.
https://projectivp.wordpress.com/category/schematics-manuals-documents/

In my opinion the HR series of amps are a really good sounding amp.
I would keep it & enjoy it.

That is an odd if not interesting amplifier circuit.

Each polarity side is basically a mirror image.

Cute 'bias mute' circuit.

Thanks for winding it down.

You need to figure out where the distortion is coming from.
From the datasheet the power amplifier has an input sensitivity of 125 mv at a rail voltage of 9-12Vdc.
That is o/k for a typical guitar signal.
The preamp is a different animal.
Input sensitivity is rated at 5 mv @ 9-12Vdc power supply.
A guitar signal will overload that input.
I would go with the datasheet suggestion of adding a 10K control to limit the input signal.

"Features:
• Operating voltage range: 4 - 13.5VDC
• Music power: 3W @ 9 - 12V
• Frequency Response: -2.5dB @ 20Hz, -0.2dB @ 20kHz
• Gain: 34dB for Amplifier, up to 58dB with pre-amplifier
• Input Sensitivity for Amplifier only: 52mV RMS @ 5V, 125mV RMS @ 9 - 12V
• Input Sensitivity for Amplifier & Pre-amplifier: 2mV RMS @ 5V, 5mV RMS @ 9 - 12V
• Quiescent current: 2mA (Pre-amplifier) + 30-60mA (Amplifier)
• Standby current: 2mA (Pre-amplifier) + 1-10uA (Amplifier)

Note: Optional 10kohm potentiometer for external volume control use RP-7610"

The Cannabis Rex speaker is a fine speaker.
I have thrown them in Fender Deluxe amps and they sing!

I find it odd that you have 'less noise' at the ground connection of U8 & U9.
There should be 'nothing' there.

The amp uses a surface mount TAS5615 Class D output IC.
As both of those blown capacitors are on each output rail, it is hard to see why both of them blew up.
Unless the output IC went belly up & cooked them at a high frequency.

The output transistors that you picked are way overkill for the amp that you are designing.
Yeah, they will work.
But still.
Way over the top.