You can also remove those resistors and use the holes for wires to run to the send/return jacks, just replace the resistor along the wires so it goes back in the circuit.
I would solder the resistor to the send jack and then add the wire to the resistor.
If it works ok then put a bit of heat-shrink tubing over the resistor.

I would have thought that the place to insert the send/return would be at C26/C126, or just before C26/C126.
I think it looks like the circuit between R19 and C26 is where the effects and distortion might enter into the circuit.
I might be wrong as I am not familiar with this amp.

G'day guzzis3, welcome to the forum.
I often drop in on the Marshall forum as well.
I don't own a Marshall, but one day....
I can't afford one on a disability pension.
I sent you a PM for your mailing information.
Well se what happens next time I go into town.

It looks good, but.........not for me, if I can't make one myself from TH parts .
However.....If someone wants to give me one I would be one happy chappy.    
(Happy Chappy is an old English saying and so is gay blade, both means a happy social person)
Just don't call a bloke in a pub a gay blade unless he was around in the 1930's.

Be very careful when tampering with the ground wires in this amp, they might be critical to the amp.
The amplifier is a "Flying Rails Amplifier"
Way beyond my experience, but the +45 -45V supply to the power amp has no earth on the center tap like conventional amplifiers.
The center tap is the output from the amp to the speaker, along with the chassis earth.
The +45 -45 voltage rails are moving up/down in relation to the chassis earth as the amplifier is working.
The center tap of the +45v -45v does not stay at zero volts in relation to the chassis, it moves up/down up to +45v and -45v.
This can make the +45v rail look like 0v to +90v, and the -45v rail can be at 0v and down to -90v during operation in relation to the earth on the chassis.

I'm confused just trying to explain it.

Schematic

The 50VA Toroidal I had ordered lasted 1 minute before the primary went O/C, I went to a local shop and grabbed a 20VA 30V CT transformer yesterday and tried again, all seems good so far.
I tried it with the 12 watt reverb circuit and mostly ok, when I tap the box I get reverb noise even with reverb pot on zero, possibly feedback through the final OP amp ?
Anyway, it sounded good through a Celestion V-type 70watt speaker I had on hand.
I built this one using 2N3904, 2N3906, TIP142 and TIP147 transistors in the power amp.
I borrowed the reverb from a Marshall G15RCD someone gave me earlier in the year, not the correct reverb tank but close enough for trying out.
At least it worked first time so I'm happy.
Now I just have to get some ply and make a cab for it, and a speaker and reverb tank.
I am also making a 12watt Bass version at the moment so I will have to see how that goes, I might have to modify it as I think the Bass version has some HP filters that are a bit too high for my liking.
I might make the cabs slightly larger then original because the original Bass cab was open back the same size as the lead cabs.
I might make the bass cab a lot deeper but the same front size as the lead and reverb cabs I am going to make for these.
Here is the Gerber file I sent to JLCPCB to make the PCB's, and the latest revision of the BOM.
I have noticed one fault with the BOM already, C24 on the 30W Bass 5503 list should be 220nF and not 22nF.
The schematic is a combination of all the 12watt schematics I found, and the power amp section is a mix of all the 12 and 30 watt schematics I found.
Just use the right combination of components and it should be ok.

I think the Lab Series schematic explains it.You cannot view this attachment.

That power amp PCB could do with some flux remover.
It would be a fun amp to trace out the schematic from the PCB's.
If you take a photo of the PCB with a bright light on the opposite side it should not be too hard.

There are some amps where the high and the low inputs are opposite to most normal amps, I can't remember which ones they are.

I think the most important thing to remember when choosing low or high input is to use the one that makes you smile, end of story.

Are you using two different guitars when you try the high and the low input ?
Does one of the guitars have active pickups and the other normal pickups ?
Some guitars have active pickups that have a battery and electronics installed in the guitars, they put out a larger signal then standard guitars with normal pickups.
That's the reason why some amps have a High and Low input (as far as I know).
One input is for the smaller signal of a normal guitar and the other input is for a stronger signal from an amplified active pickup guitar.
 This is not to be confused with amps that have more then one channel, usually one clean channel and one or more channels with distortion.

I think you will find that the amplifier is working like it should.
Does one input sound louder then the other with the controls set the same ?

 

First check the voltage on C102 in both AC and DC.
Should be approx 98V DC.
Check the voltage on the speaker out with no signal both AC and DC.
If you have a large DC on speaker out then don't leave a speaker plugged in.

Like Phatt said, send and return is the signal going into and then coming out of the DSP effects board, just like amps have send and return jacks that we can connect stomp-boxes to.
By placing the link in there you will no longer be affected by problems in the DSP board.

If you make the modification and still have problems, then its not likely to be the DSP board.

Next time you open your amp up could you please get some better photos of the board ?
even better if you can remove the sticker on the large IC so it can be identified.
It might not help to fix it but it would help understand what is happening in there.

A picture of the DSP board would be good, even better would be a part number if there is one on the DSP board.
There are some DSP boards on Ebay, also some other places on the net, but a part number or a picture would be great to identify which one is needed.

https://www.loudtechnologies.eu/shop/en/crate/dsp-effect-boards/g-10000030

Have you looked to see if you have 10v going into the DSP board ?
There might be a 5V regulator and maybe a 3V3 regulator on the board as well.

G'day Victor Van Dort
I am very confused with your post, why do you think it is impossible to repair ?
The schematic looks like a fairly easy one to read and follow, and parts are no problem to obtain that I can see.
The DSP board would be more difficult for someone with my limited experience with digital audio boards but it might be nothing wrong with that part.

It might be so simple that it is a solder joint that was bad to start with and has corroded a bit with age, simply re-melting the solder would fix that problem.
It could be dirt in a connection or plug somewhere or any simple problem that any technician would find in no time at all.

I feel that most amps can be fixed as long as the parts are available, and many parts that are not made anymore can be substituted with better modern parts.

There could be a young electronic engineer in the making among your friends or family, they might love to have a go at fixing it for you when they get a few years older and more experienced.

Do you have any electrical / electronic experience ?

Lots of pictures would be good, with enough pictures of the PCB's we can make a schematic for the amp.
with 575 watts max input and 2 x 15" speakers it must be a decent amp.