YOu mean you get nothing from the effects loop send. It would be normal to get no output from the loop return.
I'd have to look up the V tone, don;t know it. But first place I'd look would be the low voltage power rails for the preamp. I assume +/-15v, but maybe +/-12. Check at some op amp in the middle of the board. Now apply a signal to the input and trace it. I know, there are no schematics. But look at each op amp on the board and see if the output pins have DC on them, usually shouldn't. And is there signal on those outputs?
Put a scope on each output and touch the associated input pins with a meter probe or something. There should be a resulting click or hum.