1. I enable "Listen to this device". I hear the mic through my speakers, but the meters don't move.
2. I go to the UX2 control panel, and set "Hardware Meters Show" to Input, and they work. Yay.
3. I disable "Listen to this device", and I no longer hear the mic, and the meters stop moving. The mic is still active though, and I can record with it in, say, Windows Sound Recorder. So it's the same state it was in before I enabled "Listen to this device".
4. I re-enable "Listen to this device", and I can again hear the mic, but the meters don't move.
5. As I have not closed the UX2 control panel, I switch "Hardware Meters Show" to "Send 1-2" and then back to "Input", and then I press "Apply", and the meters work again.
So it would seem that the driver notices the "Listen to this device" state change and turns off the meters, meaning I have to fool with both it and the Line 6 control panel just to use the meters outside of POD Farm. Moreover, I would have to redirect the Playback target for "Listen to this device" so it doesn't come out my main speakers when I'm recording something. It would be nice if there were an easier way to enable the meters outside of POD Farm.