Hello and welcome. To answer your questions: 1- The STX II will have no problems driving the 250ohm beyers. I'ts a very capable soundcard. 2- I don't see the need for a separate headphone amp, unless you decide to buy hard to drive headphones. It will not affect the sound in any way (but you need to use the STX II as a source) it will only amplify the signal coming from it.
8/18/14 at 5:18pm