I have the dreaded hum

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.

So the phono preamp is a stand-alone and is OK.

I'm just trying to be clear here.

And the 'preamp' is full function and has a LOMC input? If nothing is connected to the inputs of that preamp, does it still hum when its in the MC position? Its OK in MM position? How about CD?

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.

There is a problem in MC portion of the the internal phono stage. I'm not knowledgeable enough to tell you if its the MC switching or or the MC gain or step up, but there is no question that there is a problem in the internal phono stage. You know that the cart and arm wiring, and the phono interconnects are OK and you know the line and MM inputs are OK on the full function pre-amp. There's nothing else left.

I had the same thing for a while, except mine was intermittent. I had to replace the Rca's. One had just worn out over the course of 25 years. I tried everything else first. Everything has been fine since. Good luck.