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?
 
Thanks!
 
 

I also have a pair of K701 that I run straight out of my Essence ST and I'm planning on complementing my setup with a Matrix M-stage in the near future. But, this is something that concerns me a bit as well, will I hear any difference? The M-stage and K701 are suppose to be a very good match according to a lot of people here so I'm really hoping that it will improve the sound.

The Essence uses the TPA6120A2, a dedicated and specialised headphone amplifier chip - it can output a not inconsiderable amount of power into varying loads and maintains low distortion throughout. The only real flaw of the headphone amp is that the output impedance is a tad on the high side (10ohms) - whilst this is not a problem with high impedance headphones such as the HD650, it can potentially have an audible impact with lower impedance models.
 
I would suggest you have most certainly not lost your marbles with the STX. It is a neutral amp that drives high impedance loads measurably superbly. The only upgrade I can think of that isn't going to most likely rely on the squishy grey thing betwixt your ears to decide there HAS to be a difference would be an amplifier which is similarly objectively excellent, but without the flaw of a high output impedance.
 
I would recommend the Objective2, at $135 built from JDS Labs (or you could do it yourself for cheaper) - you won't get verified objective performance like that at similarly low price tags (I would argue that the objective performance is so good there is no possible audible improvement if you're just looking for accuracy of reproduction)
 
However, even with the lower output impedance of such an amp and the superior damping/less frequency response deviation that results, there really is no gurantee this difference is going to be readily audible.

It's all about matchups.

Hmm, interesting... The matrix M stage has a 5 ohm output impedance and I've read amazing stories about how it matches well with the K701's. Any owners out there have direct comparisons to the STX?