It strikes me that, if all the 3k's signal path goes through the headphone circuitry, you might need to be careful to ground yourself before plugging in a set of headphones that might already be on your head or in your hands, and possibly a small static charge built up. Touch the metallic barrel of a jack plug, or the metal on your mixer, or something like that, otherwise it is possible (if however unlikely) that the discharge get into the headphone circuitry.

I know I'm getting ahead of myself, here, just blueskying possible causes...

Do you recall any particular incidents (perhaps the power going out on you while the 3k was switched on) between now and when it WAS recording alright? Strictly speaking, with an external power supply, it should be much harder for power fluctuations to damage your unit, but it IS another possibility...