Tracking down a hum.

Good suggestions but I agree with boomerbillone probably a capacitor if the tube swapping does not solve your issue.

Thanks for the repsonses. I think boomerbillone is on the right track.

Go with the tubes first. Always keep a spare set, replace when you suspect a problem, if that fixes the problem throw the old tubes away and buy a new spare set.

What you are describing, I would bet, is the 6SN7 heaters are not powering up together (one tube is heating up faster than the other) and that imbalance creates the hum until both tubes reach operating temperature.

If new tubes cause the same humming problem the next thing to check is the tube sockets and the wiring connection to them.. Sometimes tube sockets lose their grip as tubes get pulled out and reinserted. That would be would be where I start when I open the hood. The next thing I check (on a cold amp) is the continuity of the RCA jack grounds with the power inlet earth ground.

If that does not fix it, then you start swinging for the capacitors.  

Thanks for the responses. I swapped tubes out and powered up the system, same result. Looking at the capacitors, they all LOOKED okay (no bulges or anything). Their are no capacitors in the external power supply so it's something under the hood. Either tube heaters or a bad capacitor somewhere. Fortunately this should not be a prohibitively expensive fix. It goes to the shop tomorrow.

@imhififan  I have swapped out tubes, the condition still persists. I connected my headphone amp to the power (briefly) no hum coming through that I could hear. So I think it's in an output capacitor or in the tubes heaters. It isn't the tubes themselves. 
Thanks for the advice.