Mark III loud hum

[robbie rocker]

My Mark III has a loud hum when I go into the front preamp. It gets worse when I turn up the volume. However, when I go thru the back effects return (power amp) the hum goes away. I tried replacing the preamp tubes one by one but to no avail! The weird thing is when I pull the Lead Drive knob out, the hum goes away as well (when I'm connected to the preamp).

Any ideas much appreciated!!

Thanks.