The D2000's are known to have plenty of bass, too much for some peoples' tastes. I myself had to partially markl mod them to tame the bass. Agreed with the others, you probably need an amp to drive them adequately.
Actually the AV710 is a pretty good card, especially for the price. You have to use the output jack with the Wolfson DACs though.
Also, I wouldn't really recommend a Zero. Way too many QC issues and no real customer support.
I'd say just get a nice sound card with optical out for now (for a potential amp down the road) . When you're ready, then start considering new headphones or amps.
I just tried:
AV710 -> HA-RX900
Ipod 5G -> cmoy -> HA-RX900
Ipod 5G -> HA-RX900
Same results. I'll try it with some more sources later. I removed the felt driver cover, but I doubt that would could this.
I got these headphones as they seemed to be the FOTM here and on head-fi. They were also cheap enough so that I couldn't resist :p. After nearly 100hrs of burn in, I think they are really nice and probably better than my HD555s except for one thing...sibilance. Even on low/moderate volume...