I think the MC phono stage of the X1 was designed to work best with a cartridges that have
very, very low output...and I mean REALLY low.
I only have a handful of records and am definitely not an expert in phono stages or cartridges but when I was testing out the restored MC stage on my X1 I was using a cartridge with an output of 0.33mv and there was IMO way too much gain. Those very low output cartridges do exist but I believe they are rare and expensive.
I used the calculator linked below and if I entered everything correctly it seemed to confirm that a 0.33mv cartridge is not a good match for the gain of the X1's MC amp. If I recall, I believe an idea MC cartridge would be in the 0.10- 0.15mv output range.
https://www.kabusa.com/pregain.htm
Also, I don't remember if you replaced the relays on the phono motherboard. If not, start there and that will reduce quite a bit of that hum. After that my tech replaced more parts the second time he was in there and that made it pretty quiet but ultimately there was just too much gain for the 0.33mv cartridge I was using.
Good luck and keep us posted.