Either way, I think you're looking at a preamp cap drying out - Hmmm I would say that was a certainty if the hum has slowly developed over a period of time. If it happened suddenly, it could still be a cap, but I'd be more likely to suspect a bad earth.
As Dave said, without info it's prolly a techie job.
Well, well, well. Look what I just found
Look at the first reply.
And having just had a look I can now say I was talking bollox earlier!
You have a twin pot on the valve-clean control, so without the valve in, it would simply act as another volume control
Looking at where that input control is there is just one device before it, TR4.
As this is a source follower, my suspicion then would be either C19 or, more likely, the earth point of the input jacks (look for cracks in the solder).
It wasn't me!
(Well, actually, it probably was)