Agree with Pbnaudio.
Disagree with Stringreen. Hum is normal at full volume with tubes much of the time. And especially with it from a phono source
Hum is the 60Hz A/C signal getting into the audio signal. With average tube products it is going to be present.
If it is not noticable while playing even soft music it is nothing to bother about.
The issue may be a wire or a cap or whatever inside a component. Finding the problem is not easy.
Like i wrote, if it is NOT an issue while listening, then ignore it.
The fact it goes away when you switch from TT, it really is a non-started issue.
Don't worry, be happy.