if its not DC or other power issues coming from your line, dimmers are bad for adding noise on the line, than its probably a tube. Try tapping the tubes when on see if your getting any microphonics out of any of them. Try taping them quite hard to see if there may be some particles trapped in a tubes grid etc.. if you have a second set of tubes try them.
You seem to have done all the normal things we do to find hum, other than internal issues ie bad component, its most likely tubes or power delivery from your wall. maybe try an different outlet other than your dedicated lines, extension cord may be useful.