Loud Buzz in Mono

Tube line stage has mono switch. When switching on mono there is an ungodly hum happening. This is only evident when listening to vinyl through phono preamp. When switched over and listening to cd the mono switch works just fine no hum. Any advice would be great.
It looks like it doesn't have a sufficient load when you join 2 channels.
Speak to the manufacturer about defect. Maybe they'll replace the unit as part of 'safety recall':-)
I think the problem would be with phono preamp not the line stage.
I replaced a 12ax7 tube in the phono amp with a 12at7 and the hum in mono went away. The 12ax7 was tested as good so what happened here? Any ideas?
The tube is Microphonic! Deep six the tube and call it a day. If you don't mind the lower gain with the 12at7 just use that one! BTW: My phono preamp sounds better with a 12at7 and it should use a 12ax7.