Art Audio Diavolo hum

I have an old model Art Audio Diavolo.  I like it, but it just started humming on one channel.  I switched all the tubes, and it still hums on that same channel.  Not affected by the volume.  I disconnected inputs, still not affected. Plugged it in to different outlets, used cheater plug, still hums.  Hum is loud, about 30 dbs, and kicks in after 30 or so seconds, like its warming up.  Plays music, but hum sort of ruins it.  Did something burn out?  I'm the third owner, amp is over 15 years old. Thanks.