I am having a hum after setting up two EAR 890's

I have gone to two EAR 890's in mono. All connections are good. Now I'm getting an audible hum from my speakers. If only one 890!is being used in stereo, all is good. However, in mono mode/set-up I'm getting a hum. Thoughts?
Sounds like a ground loop hum.

For a test try installing a ground cheater between the plug and the wall receptacle of one of the amps. Check for hum. If you still have the hum try the cheater on the other amp and check for hum.

How are you feeding the two amps? Plugged into the same duplex receptacle, same circuit?

Or is each amp plugged into different power circuits? Please explain in detail.

Do you have a multimeter to check AC voltage?
Is your Phono lead set up for a truly balanced signal, i.e. green and Blue lead not connected to ground ?

Good Listening