Yesterday I had XM and FM both cut out for the second time that I can remember on my 2016 X, both occurrences coming post MCU2-infotainment-upgrade. I did the two-thumbwheel-reboot, and both resumed working normally.
While the issue was happening, I could select an XM channel and even see the programming info (ie what song was supposed to be playing) but there was just no sound.
As far as I can tell from this thread no one has mentioned MCU2 upgrade until now: I do recall that the reason you had to pay extra for an FM/XM upgrade after the MCU2 upgrade was something as simple as the old MCU1 expected analog signals but MCU2 could only receive digital signals and they needed to add something in the signal chain to accommodate this. If that's really the case (and if you look at photos of the MCU2 unit, the signal now comes into what looks like an Ethernet port) then I could imagine lots of ways you could have intermittent radio/network problems that go away if you just reset everything with a reboot.