Sumo Nine Hum

Hello... First post on this forum.

I have a sumo nine that I love. I have an odd issue.

If I turn the amp on with no input there is a loud hum out of my right speaker. If I connect inputs, from a preamp, and turn the preamp on the hum goes away. Is this something I should be concerned with? It would seem that there is a ground issue internal to the amp and the right channel is using the ground of the preamp?

Also, both sides of the amp are within 1-2 degrees Fahrenheit of each other and it is running really cool ~92 degrees after an hour of listening. Is this normal?

Thanks for your help!
 
Yeah... I noticed it when I turned my preamp off before the amp. Seems fine and sounds wonderful when there is voltage on the inputs. I may take it apart and check for a loose ground wire. I had checked all the caps and bridge rectifiers before turning it on (it had been sitting for a couple of years).