Why my SUT hums only with certain phonostages ?

I use a vintage Ortofon STM-72 SUT with my Lyra cartridge. Curiously, when I connect it via my solid state RCM Sensor prelude phonostage, there is a strong hum. The hum lowers when I connect the earth lead of the phono cable to the phonostage earthing point, but still there is a hum which is distracting. However when I connect the SUT to my Shindo Aurieges MM phono stage there is absolutely no hum. I dont even have to connect the earth lead. What is the reason ? Is it my SUT or my phonostage ?
Maybe there's nothing wrong with any of it. Have you tried different settings on your RCM? (I know I probably don't have to ask that question given how nice your gear is, but its the first thing that comes to mind.)
Hi Zd542, I have set the RCM to its MM configuration (47kohm, 55db).
Pani I get a low level hum from my ear mc4 into the mm input on my nagra vps...with allaerts mc 2 finish. It's a low out put .2mv....

When I use my lyra atlas...same sut and phono no hum...but I also don't need so much gain...so I suspect the phono has a noise floor that the allaerts exposes...

I do prefer the sut vs the mc input on the nagra

I've tried all sorts of grounding....wish I could directly answer your question
Have you tried swapping in different ICs from the SUT to the RCM? Probably not the solution but worth checking. I have several sets of perfectly nice, unshielded ICs that cause an awful hum when placed between SUT and phonostage, although they're silent anywhere else.