Call Kustom & ask for a schematic.
https://kustom.com/contact-us/

Pin 1 & 8 of the LM386 set the gain.
You are supposed to use an e cap (10uf) for more gain.
Not a jumper wire.
No cap and the gain is set to 20.

You will have to install something to limit the gain.
Some of the Zoom patches are off the wall high gain.
I have often wondered why they chose to do this.
You go from one one patch to the next one in line and the gain is off the charts.

Or, if the FX loop is not being used, you can simply leave a patch cord installed.

I would run a patch cord from the FX Send jack to the FX Return jack.
The Return jack has Normally Closed contacts that if they get dirty will prevent the signal from passing through to the power amp.
As to a schematic. Have you tried contacting Rocktron? techsupport@rocktron.com

Nice.
That may be the answer to the Roland EP-7 ribbon cable failure fiasco.

There are a ton of prebuilt low power amps on E bay.
Search TDA2030 amplifier.

Keep in mind that the TDA2030 has been out of manufacture for a number of years.
The items currently on the market are knockoffs.

5 pin Mosfet???
Sanken made 5 pin darlington output transistors.
The one that comes to mind is SAP09.
There also was a 15.

"Tube Logic – a marriage of solid-state analog technology and DSP"

Not too sure how the schematic will reveal the true nature.

I am no expert on the LM386D amplifier IC but it seems to me that adding a 'tone control' circuit on the output of the device is not a good idea.
Have you tried the circuit on the input side?

Some type of schematic of what you have tried would be a help.
The tone control is only 5 components.

You can even 'build' it on to a control pot.
http://beavisaudio.com/techpages/BigMuffToneControl/

Here is the schematic of the actual Big Muff TC:

The transformer in the schematic specifically states "Export Model".
That is not what the OP has.

Here is the schematic

Try this:

Who have you 'tried'?

I would suggest Part Is Parts. 1-800-590-0014 USA/Canada
1-802-365-7257 International

And/ or Full Compass. 800 356 5844

They will do there best to get the part from Marshall.