Soundcards may be more at risk to being effected by electrical interference but it doesn't guarantee it. Different cases and different components will result in plenty of different outcomes.
I'd imagine the Xonar D2X 7.1 is expensive mostly because it's a 7.1 card but it's not aimed at providing the best stereo sound quality possible (Essence STX and Titanium HD are the cards aimed at that afaik)
Yes I understand this. A friend of mine has an STX so I have heard it many times, haven't heard the Titanium though. I just picked the D2X as an example of a modern card that I have tested in my rigs in the past couple months. I have heard lots of others too although they all tend to suffer from the same issue(s). I have tested in 2 different PC's both of which have high quality PSU's (I seem to remember the D2X drawing power from the PSU). I don't as a rule by cheap components, I appreciate and buy quality.
Speakers I am using are PMC TB2i (space limited), with a Bryston 4BST/Classe CP50. Headphones are Audio Technica ath-ad700 and Beyerdynamic AMS-DT-990-Pro.
Now, what DAC's have you heard to compare your sound card to Sin_Chase?