I use to put some stuff in the back of the unpowered KPA, including one short cable for my Frfr monitor.. today I forgot to unplug the cable from the monitor output and the tip of the cable was touching the metal back panel. Headphones in, guitar in, a random high gain rig in, and suddently a very loud feedback kicked in. I first thought: this is nuts! Never heard feedback from headphones! Than I disconnected the cable and everything was alright.
Hope this helps