UR12 headphone hiss

I use a UR12 for voiceover work, because I can record directly into my iPad. My entire studio fits in a padded briefcase. But I’ve noticed that the headphone output is low (I’m using Sony MDR-7506 cans, also have Senn 598 – neither are high impedance), so I have to crank the output knob.

Anything above 12:00 on the knob gives me increasing hiss. I don’t believe it’s on the recording, just in the cans. Is this normal? Any solution? Would an outboard headphone amp help? Thanks, all.