Late Night Hum Issues: Help?

Some nights, especially on the weekends, I'm getting an annoying hum out of the phono stage, which is a Pass XP-15. But I had the same problem previously, with an XOno, so it's not the phono stage as such. If I disconnect the cable from the arm, then there is no hum, so I'm thinking it's probably that the cartridge is picking up radio frequencies. I live to the south of Boston, and there is a radio transmitter not far from here. (For those who know, I live a mile or so south of the Great Blue Hill.) Perhaps some AM station boosts its output and that's what I'm getting? I should add that, even during the day, I get some of this same hum, but it is much, much lower in level. That is why I don't think it's a ground loop or anything like that. That wouldn't change so much.

Oh, and if I touch the tonearm at night, it increases the hum. Disconnecting the ground wire has no effect.

Anyway, this is annoying, and I'm wondering if anyone has any ideas. The cartridge is a Shelter 501 Mk II. Is it worth trying another? Would it be worth trying an MM cartridge, on the theory that less gain would mean less hum? Or even a high-output MC? If so, does anyone have any nice suggestions for such cartridges, in the same price ballpark as the 501?

The phono cable itself ought to be well shielded, and I've tried a few different ones without real change. What about trying ferrite core thingys on the phono cable? Would that have a chance of filtering the radio stuff?

Any other ideas?
I don't think RF causes hum. If your cables are unshielded, you will actually hear the radio stations as your cables act like antennas. You need to make sure you are using properly shielded cables and that there is nothing wrong with your ground wire. Another trick to try is to reverse the polarity on your turntable AC plug as well as your preamp.
Check your connections to the cartridge headshell and cartridge. You may also have a broken lead wire but most probably a poor connection to the cartridge/headshell. (Also check connections to pre-amp). IMHO, its worth a try.