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.

If you are very very lucky it might be so simple its just a bad connection in the tube socket.
Pull the tubes out and reinstall them a few times just to make sure, if you have some Deoxit give the sockets a spray.

Just remember that there is high voltages in tube amps and the capacitors can stay charged for a long long time, be very careful so you don't damage the amp when you get knocked across the room from a charged capacitor.

The only reference I have seen for that Zener is that it is a medium power zener diode, whatever that means.
http://www.wylie.org.uk/technology/semics/GEC/GEC.htm

If you have a 1 watt 13V zener then try it, check how hot it gets.
If you have to buy one then why not get a 5 watt 13v zener diode, it will look chunkier and that it belongs in the amp.
You can get 5 watt 13 v zeners for under $1 if you shop around so that is what I would do.(Mouser)

I don't think my copy of the schematic is any better on the power amp stage.
Can you take a picture of the PCB so we can see the board and components ?

Disconnect speakers before any testing
The first thing to check with power off is the bridge rectifier with a multimeter set to diode checking.
If the bridge seems ok then check the power transistors for shorts from collector to emitters. (Q110-Q113)
Check the power resistors as well (0R5 5watt).(R123 - R126)
Power on and check if there is any voltage on the + or - 63v rails to ground (0V).
Check for DC voltage on the speaker output
Do this quickly so you don't have the power on too long,turn power off and check if any of the power transistors or power resistors got warm or hot.

Hopefully one of the more experienced guys will give you more/better advice.
Cheers
Mick

The collector on T12 should not be +30.8v, it is connected to the negative rail.
Are you keeping the common (Black) lead of your meter on 0V (ground) all the time ?
It is very important to clearly indicate the positive and negative readings so the experts (not me) can work out what is wrong.
Maybe worth re-measuring the voltages ?