Tube amp troubles


I am using an Ayon integrated tube amp. During a recent move, I had to take out all the tubes and when I put them back, I mixed up the low-power pre-amp tubes. I’m only mentioning this is as a context - I don’t think it has much to do with the actual problem I have. 

When I switch on the amp and start using it after some warm-up (the amp indicates when it’s ready for a load), everything works fine for about an hour. Then, the left channel goes totally mute. If I switch off the amp and let it rest for a few minutes (but not long enough to cool down entirely), it works again (after powering it back on) for a while - and then the same thing happens. The problem occurs regardless of the input, volume, etc. 

If you have any idea what this could be, that would be very helpful - I would like to avoid shipping this 48 kg beast to the only authorized dealer.

Also, if you know of anyone in the greater DC area who repairs tube amps, that may come handy, too, in case there is no quick remedy.

Thank you all,

Switch the "low power pre-amp" tubes from left to right and see if the fault follows.
Have you switched all the tubes from one side to the other, to see if the problem follows the tubes?
I agree with all suggestions. A tube swap out is usually the first step to see if the issue "switches" channels. If it doesn’t switch channels then something internally (not likely the tubes) is going on. Then possibly a fuse but it does sound like a thermal issue. Some of the technically oriented here will know what to do next.
Thank you all. Should I test both the low power and high power tubes (in separate rounds)  by switching sides?
Ok, I switched the input tubes but the problem remained on the same side. So it’s not the input tubes’ fault. I will now try the output tubes (although they all have the same glow and similar temperatures).

I wonder what other component may react to the gradual build-up of heat with switching off one side of the amp. Also - and please do forgive the question - is there anything else that can accumulate during use in the components of a tube amp? Something that would only trigger, say a fuse, after 20-30 minutes?
Sure there is. You could have a damaged passive component that is overheating or out of spec such that it is breaking down enough to trigger a protection circuit. You could open it up and do a visual inspection but it is unlikely you'll see anything. However, if you do not know what you are doing, it's best to have a pro look at it. 
Take temperature measurements from the output transformers and see if they are close.  
Thanks again. Last question:
do you know of any professionals (preferably within 100 miles from the greater Washington DC area) who could look at the amp?