I would go with the Fender Champion 100.
2x12, FX galore, voicing.
And they sound just as good whether played soft or loud.

To try and prove whether or not the power switch is the issue, turn the power switch to On & then plug the power cord in to the outlet.

Separate & do what with the board?
There are gut shots here of the chassis.
https://amploft.co.uk/blog/2011/11/30/trace-elliot-speed-twin-c50-repair/
What are you planning on moving?

As far as I can tell that unit is a 'Constant-Voltage' amplifier.
ie: A Public Address amplifier.
Yes, it needs a preamp of sorts.

Typical, run of the mill, inductor tolerance is +/- 20%.

Those voltage readings are not good.
Especially the -18 volts on the output bases.

The "Low Ripple" output utilizes an LC circuit.
The "Normal" output does not use the 'L' (inductor).

It is great when an amplifier or an effect brings out your guitar.

Which amplifier are we talking about?

Do it right!

What is important is the follwing 4 voltages.
+22Vdc
-22Vdc
+37Vdc
-37Vdc
These are the rectified dc voltages that the amp needs to function properly.
Please list them.

Here are the files.

https://reverb.com/item/78914907-peavey-705-18713-bandit-65-power-transformer

Your serial number indicates a manufacture date of June of 1975.
The schematics that you found are from 78 & 79.

I think your meter is trying to average the AC voltage on the two inner pins.
Try with the meter set to to Volts AC.