I've searched a lot about this topic and i'm getting mixed results. Let me describe my experience so far:
I currently own a pair of Denon D2000's, AKG K701 and Grado SR225is. My current setup involves the STX line out to my NAD C165 pre amp and I use the headphone out on the NAD to power my headphones. I've compared the NAD headphone out to the STX headphone out and noticed slightly more bass slam in the NAD. Everything else was identical.
I used to own the HD650's. During that time, I also had the Woo Audio 6 with Sophia princess. Shockingly, it was identical to the STX (soundstage, bass and clarity). It had more distortion as expected but that wasn't my complaint. What I didn't like was that a $200 sound card could match (I won't say beat even though I preferred the STX overall) an $800 amp that, according to reviews was supposed to be amazing in every possible way with high impedance cans like the HD650's.
Now I know many of you might disagree with me and I'm with you. I'm finding it really hard to believe what I just said above. I would love to believe that a nice dedicated headphone amp that costs EVEN twice as much will perform better than a sound card.
What can I be doing wrong? I'm thinking of buying a LD MKV for the K701. Does anyone think I've lost my marbles about the whole STX matching amps that cost 4 times as much? Could anyone that has compared the STX to other amps shed some light?