Hi
I am not sure, if the schemes match the C3 board, but I know them all - more or less by heart
Especially the one drawn by Joachim, which is the basis for the TT boards (as I said before).
If you plugin the guitar to the fx return you should have low volume, but I think it should not produce any hum. If it does and it sounds like mains voltage hum (50 or 60 Hz) then I think there is a wrong ground connection on the fx return - you could check the preamp by connecting fx send to a separate power amp (or connect it to fx return of the SLO100 you got); how does the preamp sound?
The optocouplers can be checked pretty easy with a multimeter - if you refer to the layout the connectors on the "tube" side of the board are the resistance part of the couplers as the opposite side of them is the voltage connection.
If you switch to clean then VR1 (according to SLO 0.7 scheme of Joachim) and VR4 should measure only a few ohms (I do not have the data sheet at hand, but Vactec should measure around less then 100 ohms), VR2 and VR3 should be around 50 meg ohms I think.
If you switch to lead the other way round: VR1 and VR4 are of high resistance as V2 and VR3 are of low resistance.
Everything around a few kilo ohms when switched will be too much (also check the voltages at the optos … not sure about the Vactec, but they need 1,5 to 2,0 volts in order to switch correctly I think).
If the preamp is working correctly (as mentioned you could check this by using fx send) the power amp may be wired wrong - how is it sounding if you adjust presence and (if installed) depth?
I still assume that there is a mismatch in the parts labeling of the C3 board and the schemes you used - I will check it tomorrow as I have got a board, too (as mentioned).
I will have a look at the pictures
Regards, Stone