Mark III Red Stripe Combo Graphic EQ and Reverb

This is my first posting on this board.
Anyhow, my amp went really quiet the other day. I usually play the clean channel, and rarely above 2 on the input volume.
I have to crank it up to 9 to hear anything, it's acting like it has a built in attenuator.
If I run a guitar cable between the send and return jacks... no change at all.
If I plug my guitar straight into the return jack, I get no sound.
Swapped all preamp tubes one at a time.. no change.
Tried different EL 34s..no change.
Cleaned all the various jacks.. cleaned the graphic sliders...no change.

Take it to a qualified amp tech. It is probably a mode switching problem.

As long as it's there, see how old the power supply capacitors are. Odds are that they've never been changed, which means THEY'RE DUE. Change them before they fail and take out a transformer.

Do you have any idea which component is faulty?

Epilogue:
Just hot the amp back from the shop and it sounds glorious.
It blew a plate resistor on the V2 preamp tube.
Thought I would share to help some future soul with the same issue.
Thanks to all who responded!