Troubleshooting Hum in a Tube Guitar Amp
There are many different ways that you can get hum in an amplifier, and its hard to troubleshoot without the amp at hand. But generally; hum can enter in three ways:
Directly, inductively or capacitively coupled.

Looking at your case; did you try to short the reverb return input  at the amp when disconnecting the reverb. If the hum is still present, the problem is in the V4A stage. (shorted cap etc...). If not, you could have an open R28, a bad cable or a broken tank. The tank would be most likely of the three. (Yes, the tank can be faulty and still give sound.) 
Did you try to connect the send to the return directly? What did you get? 
Even if the hum is still there with the tank disconnected you still have an open input that would pick up hum capacitively. This would be the same as a corresponding fault in the tank or the cable.
Good luck!