[Ryvius]

I am currently using a pair of Senn hd558 and Shure 840, and I find myself spending more time with the hd558 as I like the sound from an open headphone. Both are currently connected to the Xonar DX, and planning to upgrade to the Xonar Essence Stx. I have my eyes set on the DT990 600 Ohm and the AD900X right now because they are on sale locally for under $280cdn. If I spend most of my time listening to jpop and watching movies with the headphones, which one would be the better choice?  

 

[PurpleAngel]

Just get an external headphone amplifier, like the Schiit Magni headphone amplifier, $99, and plug it into the Xonar DX's "front" port. and plug your headphone into the Magni.
The Magni on it's own should improve the audio quality of your current headphones.
Also as the Magni's output impedance is less then 1-Ohm, it's better for driving your current headphones, over the STX's head amp's 10-Ohm impedance.
The AD900X should be a little better for music audio, over the DT990.
Also the Magni would be preferred for driving 40-Ohm headphones, ike the AD900X.
The DT990 are great headphones for listening to the Blu-ray audio (Movie theater like sound).
 
You did disable the motherboard's on-board audio? in the BIOS, when you installed the Xonar DX.
Try the third party Unified Xonar Drivers.
and try using Foobar2000 for playing audio files.