Xonar Essence STX or Cheaper Soundcard???

I'd like to get a soundcard to up the performance of my music collection and gaming experience. Currently have a set of Audioengine A5 stereo speakers hooked up to onboard sound, and i feel like with a soundcard i could get more out of them.

The Essence STX looks awesome, but $200 seems pricey. how would the Xonar DX match up with the essence for a 2.0 stereo system and sound quality???

All my music is in MP3 format and is from 256-320kbps.

I sometimes use headphones (Audio Technica AD700), But since they are low impedence i figured i didnt need the amp that is built into the Essence STX.

So, what do you gurus think? Worth the extra money? Will i see a noticable improvement with the Essence over a cheaper ASUS Card???