Not having a JMW 9, I can only offer my generic response, which is that it sounds like either (1) something went wrong with your cartridge internally, or (2) one of your tonearm cables has suffered a short to your arm tube.
To check #2, I would visually inspect the tonearm cabling between the cartridge and the tonearm, to make sure there is no inadvertant metal to metal contact. Also, I would wiggle the tonearm cable at the back of the tonearm (the one that supplies the antiskating compensation), while holding onto the arm so as to generate the hum you described. If the wiggling of the cable causes the hum to disappear and reappear, then there may be something wrong with that cable or its connectors.
To check #1, I would switch cartridges, if possible. If the problems go away after you switch to another cartridge, be sure to switch back to see if the problems return. (If the problems don't return, then maybe the cartridge is OK, and it was just a matter of a tonearm cable shorting out somewhere.) Good luck.