If you bought a PCB for a pedal you can still run leads to the pots and mount them in a case like a real looking amp, just use some supports under the PCB so it keeps steady.
The TDA7297 will give you about 12 watts at 9V or 14 watts at 12v, you can get a TPA3116D2 MONO amp module fairly cheap that can run off an old laptop power supply.
I have a 3116D2 mono module in a 125B box with a tiny fan that I run from a 19v laptop supply on my bench, I can give it 26 volts and get 100watts from it.
It looks just like a small stompbox and will run on voltages from 4.5 volts to 26 volts.
15 watts from 12 volts if you are camping.

https://tagboardeffects.blogspot.com/2016/03/sunn-beta-preamp.html
https://aionfx.com/project/beta-preamp/
https://dirtboxlayouts.blogspot.com/2020/08/sunn-beta-pre-amp.html
https://pcbguitarmania.com/product/sun-beta-lead/

If the crunch is developed in the Power Amp then you are out of luck, unless you have lots of thick blankets.
I don't know enough to go into that but if you can check what the waveform looks like at the 1uF capacitor after the Tone pot but before the 2SC536F you will find where its coming from.

If the voltage is right at that 1uF cab you could add an effects loop there I think or even another volume pot, but I can't see it doing anything that the current volume pot doesn't do.
I would do what Phil said

This might be of some interest: https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/

Re you sure you want a clean preamp ?
I though the SUNN Model T was more suited for grunge, doom, stoner, drone, sludge type of music.
Then again that is what most people use the SUNN BETA amps for but they can put out some nice clean sound as well if set right.

Try looking at lots of SUNN BETA youtube clips to see what you think of that one, it can sound similar to a SUNN Model T.(Try to find a clean sounding clip)

SUNN BETA amps are all IC's with a couple of FET's for switching channels, they get a bit more complicated but you can buy PCB's for stompboxes for one channel that can drive a power amp.
You can even find veroboard layouts for them if you look around, I am thinking about making a PCB to fit one in a 12"x3"x8" Hammond case with a class D power amp in it for a Bass Amp. (500watt @ 4ohm)

I was looking for a schematic for a Marshall 5501 Bass amp with the JM79A PCB and I came across this "Google Drive Box" with lots of schematics to grab, you can download each separately or download everything at once.
I started with each schematic but then I switched to just amps,effects, mods, guitar and bass, others.
You will be there forever if you do them separately.
Grab them while you can.

( I think I was actually looking for the 5503 30w bass amp schematics)

https://drive.google.com/drive/folders/12p6GLcMMEupxIvLF7XXDQS88ZUuRWRi4

Is that a 50k-B volume pot (B50k)?
If it is change it to a logarithmic pot for better low volume control.
A50k is what you want for a volume pot, it should make it better.

I would find clips on Utube and other places, read forums etc to find an amp I like the sound of, and then go from there.

I made a PCB for the Marshall 12watt amp because its popular and sought after, I posted the gerber files somewhere on this site for the PCB files.
If you are just after the pre-amp then you could easily build it on a Solder-less Plug-in Breadboard to try it out, change the tone stack and gain components etc.

How complicated do you want to go ? A simple pre-amp or multi channel pre-amp ?
What sort of power are you going to run it at and is it for home or are you going to use it in a band etc ?

 Diary Of An Axeman:-
I forgot to say that I love to see photos of the insides of amps and pedals, if you could find it in your heart to open the pedal again and take lots of photos of the front and back of the PCB you might get other people to jump in and trace the schematic for you.

We just need to see the resistor bands and capacitor values etc, it can be a fun exercise to do a trace on old gear to get the schematic for it. Sometimes it's amazing how many variations there are out there.

I have done it a few times for SS amps myself.

Cheers
Mick

I saw a schematic that someone traced from an original unit and it showed a 2Amp fuse.
I think that seems a bit large for a tiny transformer.
If it should be a 2Amp fuse it would be fast blow.

If I had any gear from the late 60's I would test and most likely replace any Electrolytic filtering capacitors in there.
At least test all caps to make sure they are still within specs and not shorting out.
If the 1000uF cap is playing up you could do some damage to that little transformer.

Even better would be if you traced the whole pedal and left a schematic here of what you find, there are a few variations out there with different value components in them.

Hmmmm....1968-1969,,,,, I think that's when I first started plucking strings on a nylon string guitar.

Thanks g1 for pointing out that error in the file naming, I did not realise that that C was for the revision, I think I have fixed it on post 10 ok now.(and in my computer as well).
I would love to see revision E if you have it g1, it would be great if you could post it.

700RB-11 Manual page 10 shows the 10Uf 50V NP capYou cannot view this attachment.You cannot view this attachment.You cannot view this attachment.

I noticed that one variant of the 700RB-II power amplifier uses a 10uF 50V NP capacitor where your C21 is, perhaps your model should have a NP (NON-POLARITY) capacitor in there.
If the cap is polarized and gets fed a reverse voltage it would most likely smoke and pop like yours did.

Can you trace where the track goes to from pin 8 on the LM3886 ?
It most likely goes through a resistor to the negative supply voltage somewhere. (Mute 2) ??

Do you have the preamp schematic for the early 700RB ?

If that is the correct schematic then Q6 is in a totally different part of the circuit, and J1 shorts the B & E legs on Q6.
Is Q6 in the main power amp section or in the circuit with the LM555 IC (U1)?

Looks fantastic, nice tolex work. Is it red jungle bark tolex ?

Nicely done.

Built what exactly