I have the dreaded hum

I tried a diff set of interconnects, no change.
The dealer is sending me another pre-amp to try, but he still thinks something else is causing it.
Could it be that the extra phono stage is isolating a ground loop?

Atmasphere. When I use the stand alone phono stage the hum is gone. I only have hum when pre-amp is on phono input and switched to MC. That engages the internal step-up in the pre-amp. No hum with any other inputs.
All signs say it is the internal step-up, yet when I sent it back to the dealer he said he had no issues. The pre-amp was sent back to the manufacturer a few months ago for a different issue, came back repaired and apparently they checked out the phono section as well. At that time I did not yet have a TT so I didn't notice the hum. A few weeks later my brand new TT arrived and I have had a hum right from day one with the TT.
I will wait and see if I have any issues with the replacement Pre-amp the dealer is sending.

To recap, i have a pre-amp with a built-in phono stage with a MC input.
When I connect the TT leads to the MC input it hums on one channel. It does not hum when the leads are disconnected. It does not hum on any other input incl
MM input. I went out and bought a stand alone MM/MC phono stage. When I use it connected to aux input of pre-amp it does not hum.
Thanks for your help. This is my first journey into MC cartridges.

I would be surprised if it was the RCA's since everything is brand new.