Hum in Unico SE

I have a Unico SE hybrid integrated amp that suddenly (sort of) developed a hum. I've removed all inputs except the speakers and the hum is still there.  I picked up and separated the bi-amped speaker wires but no change.  As I added back individual devices no change.  What do I try next.  I don't have replacement tubes to try but I will get some.  Also, what would be best tubes for replacement.