Nait xs-2 headphone-problem

Hi!

I have a question regarding matching nait xs-2 with Sennheiser momentum headphones. If i plug my headphones in to the xs-2 it sounds terrible, distorted. I tried another nait xs-2 with the same result. I tried different headphones with 32 ohm instead of Sennheisers 16 ohm - and it sounded great. I then tried plugging the Sennheiser to a Atom via 3,5 mm - sounded real good! Then as a final test i plugged my cheap in-ear-phones to the xs-2 and it sounded pretty good. So what is the problem? The xs-2 isnt broken and the headphones seem to work just fine plugging them into Atom or a mobil device. Why dont they match?

best regards

Andreas