I am pretty sure that any distortion or growl in Rods preamp is produced before the master pot, you can just turn the master down and crank the volume pot and you should be able to vary the signal with the master pot.
If you want to add a switch for a lower setting place a resistor in series with the master pot,  between M1 and the master pot and use a switch to short the resistor out for full volume.
I would try a 50k resistor to start with and then increase it later if you want less output.

I would place the loop between R55 and C33, that way you have resistance on the OP-amp side.
I myself would remove R55 and mount the wires in the holes where R55 was, then I would place R55 in the middle of the wire that goes to the send plug.
You don't have to cut any traces if you do it that way.

Do you have access to another Amp?
If you do try connecting the send to the other amps return to see if the pre-amp is good, and then connect the other amps send into this amps return to see if the problem is in the power amp.
You could try the guitar into the return jack but it would be very low output, that would test the power amp by itself.
The best place to start checking might be the power rails, the +-15V and the +-37V.
Sometimes the main filter capacitors can dry out when not used for a long time.

It would be better if you added your location to your personal info, there have been times when someone had one and was happy to send it if the recipient paid the postage.
I remember one time it turned out they lived in the same town so there was no need to send it in the mail.

Have a look at the last layout for the JC-120, it shows the layout of the same PCB I think.
The JC-120 appears to be 2 lots of JC-60 amps in one box.
This is the closest I have on that amp.

I would say the problem is with the Pre-amp, the reason I asked you to try the pedal is to see if you got the sound you expected.
I think the best way to go forward would be to inject a clean signal and check the test-points with an oscilloscope.
Being a SMD build it gets a bit harder to check and replace components.

The safes way to do this is to add a socket behind the speaker that the speaker goes to directly.
Also a speaker out socket on the back of the amp chassis, then you need a short speaker jumper cable with a plug on both sides to attach the combo amp to the combo speaker.
Edit: I have done this to some of my combo amps as it turns them into a separate head and separate speaker cab.
Sometimes I want to play the Marshall through the Fender speaker or the Sunn speaker, I just un-plug and plug into another combo for the sound I want.

Just remember that big power tubes can get VERY heavy, just pull one out when it's hot and try to hold it in one hand, I bet you can't hold it for too long.
But everyone has to learn that certain things are hot the hard way, I am constantly reminding myself that the soldering iron is too heavy to hold on the pointy end. (The hard way)

When you plugged into the effects return with the guitar did it sound nice and clean ?
When you plug straight into the return jack you are bypassing the whole preamp and all the controls associated with the preamp.
A preamp boosts the guitar signal up a lot for the power amplifier, that's why a guitar straight in to the return is not very loud.
Do you have another amp you can use ?
Plug the send of another amp into the return  on the Fender and you will be using the preamp of the other amp and the power amp of the Fender.(guitar into other amp)
If that works then the Fender power amp is fine.
Try an overdrive pedal straight into the return and you are bypassing the preamp again.
You have to try to determine if the problem is the preamp or power amp.
Preamp is guitar input on front to the send jack on back.
Power amp is return jack to speaker out.

I have a few quick questions for you, do you want to keep this amp as close as you can to original or do you mind changing some things ?
I personally would do the following if it was mine.
1: Change the mains cable to one with earth and get rid of that deadly earth switch.
2: The power resistors should be mounted as close to the PCB or power transistors as you can.
3: Remove the power transistors from the side of the chassis and mount a heat-sink inside in front of the fan and mount transistors and resistors there. the airflow will help a lot. The heat-sink should be right on top of the stud sticking up of the floor for best positioning with the fins towards the fan.
4: make the fan variable speed with a speed controller, if it is DC then Joecool85 has a nice little speed control circuit in here somewhere that would work great.
5: make a hole and put a grommet in it so you can mount a large 3 spring tank in the bottom of the cabinet, or mount 2 RCA sockets on the chassis so you can plug the reverb in from outside. (you might need insulated sockets so they don't short out to the chassis.

Its looking great so far.

I thought that the Model 134 was a combo with 4 10" speakers in it, if not then maybe someone has modded it to be a head.
I was going to suggest you place the reverb tank in the bottom under the speakers but I guess that's out if you only have a head with no speakers.
The combo is supposed to be 25" wide so I thought the amp chassis would be at least 23" wide.
Would you mind posting some pictures of your 134 for us ?
There are not many pics of Model 134 amps that show the insides properly.
(And we love amp porn,...I mean Amp pictures)    
Cheers
Mick

I sometimes look up tanks at https://www.amprepairparts.com/reverb.htm if I don't know the right part for an amp.
They only mention 2 different tanks for any Acoustic amps, the 8BB2D1B tank as an Upgrade for the 1BB1D1B reverb tanks used in Acoustic™ amplifiers from the 1970s, such as the  models 230 and 330.
The other one is a  8AB2D1B Also used in the Acoustic™ G60T (Model 162 or Model 163).

Personally I have no idea what went into those amps, but since you most likely have to drill new holes to mount it I would go with a large 3 spring tank myself.

The slave/direct inject is a signal out for a separate power amp or mixer, you can use this to go to another amps return socket.
The effects send and return plug is a single stereo socket, you need to use a "Y"splitter cable to use this.
Stereo plug to 2 mono sockets.

The slave/monitor is the same signal as the send part in the effects output.

The stereo plug will have send on the tip or ring and return on the other tip or ring, I am not sure which way around it is.

The 2098 was the head version and the 2200 was the combo version of that amp.

You can turn the speaker selector to off, or just leave the speaker disconnected as far as I know, its a solid state amp so it should be no problems.

I'm not 100% sold on the idea, the effects send signal can be too much to plug into the guitar input. If you keep the level down in the Orange it should be ok maybe.
If your Roland Cube has an auxiliary input then that should be able to handle it ok
If you have 2 inputs the normal one can handle 56mV and the overdrive one can handle 10mV so use the normal one if you do try the guitar in socket.

That one looks good to use, If you look on Ebay or similar you will find many stud mounted diodes you can use, just make sure you get one with the correct orientation.
The stud mounted diodes are still used but mainly in industrial situations where higher power is involved.
Old car alternators used to have stud mounted diodes in them as well.
I have replaced some that had a 16mm thread on them when I worked as a maintenance electrician for a large manufacturing company.