'68 Marshall head sounds like an octaver, notes are doubled about a 5th note lower...

[Iplayloud]

Any note I play is doubled with the same note about 2-3 tones lower.

Even at bedroom volumes, amp is 100% mint with red ink on every soldering. (Just to say it wasn't modded to do this)

It definitely needs a cap job because it hums really bad but is it possible the bad caps would do this too, or could it be the OT or something else ?

[pdf64]

Almost certainly caused by bad power supply caps.
1st step is to replace ALL electrolytic caps - high voltage B+, bias caps, cathode bypass electros.
When that's done the problem should be gone.
Pete.