Another "kinda-newbie" head scratcher

It might be radio frequency interference (RFI) getting into the phonostages either through the powerline or directly over the air.  The source could be nearby powerful transmitter or something in your home.  You can try turning things off in your home.  I know someone who had such a problem who found out the source was a string of LED lights his daughter put up in her bedroom.  This was a big distance from his system and on a different power circuit but it still broke through.

If you cannot kill the source, you can try moving components around to see if a different position of the phonostage or wires going into it will reduce the noise (something may be acting as an antennae and these are directional in their sensitivity).

Some solid state phonostages are particularly sensitive to RFI because it overloads them.  This might be reduced by increasing the loading setting.  That is done by selecting a LOWER value loading resistor, such as 100 ohms instead of 47k ohms.  A friend had noise breakthrough that was cured by such choice; the default loading by the manufacturer was 100k ohms and this had to be changed by going into the unit to change switch settings.

You could try cleaning your connections, worked for me.

It is RFI. In addition to other suggested cures, buy some female RCA jack plugs and block all unused inputs and outputs on your phono and linestage. They’re cheap. To be more clear, you need to buy male plugs to block the open female in and out jacks.

No matter what Phonostage I use, no matter which cartridge or cables

so the only constant is your amp?