It seems this thread has gotten a bit off-track from what the OP was apparently asking, so allow me to try to get us back on topic.
@Mutsu, you were wondering if you should upgrade your Xonar D2X to the Essence STX or keep the D2X and instead buy an external headphone amp for about the same price (presumably $100-300). To help answer your question I have been doing a comparison for the last 5-6 hours of the following two setups:
1) Xonar HDAV 1.3 -> Corda Opera amp -> DT990's
2) Xonar Essence STX -> DT990's directly through the STX's Headphone out
Now I realize that setup #1 is not the same setup as what you have, but it is as close as I could come. From what I have read, people who have heard both the HDAV 1.3 and the D2X generally agree that the HDAV is the better sounding card for 2-channel music, which doesn't surprise me since the HDAV sounded better to me than the D2 (a PCI version of the D2X) that I used to own.
Additionally, I'm fairly confident that the Corda Opera (which FWIW retailed for $750 right before it was discontinued last Fall) will most likely match or exceed any other headphone amp you can find for $100-$300 (your budget), given that many people on these forums who have much more experience than myself and whose opinion I trust consider the Opera to be one of the best sounding SS headphone amps ever made.
With that in mind... Here are my notes from this comparison:HDAV+Opera - Mid bass is stronger but it's not as detailed and deepest bass extension is missing. More mid/upper treble presence, but what's there isn't as detailed. Soundstage is taller, but not as deep. You're sitting a couple more rows back from the music than the STX setup. Imaging is a semi-flat curve in front of you. More laid back, Sounds like I'm listening through a slight fog in comparison.
STX + HP out - soundstage deeper but not as tall, imaging improved, more details across the spectrum, clearer, more air, longer decay, instruments are closer to you and have more weight, 3D imaging better, better PRaT, stronger bass impact, more bass extension. Upper-mids/low treble hump, making trumpet bleat slightly. There's a level of realism here that is not present in the HDAV+Opera combo.
As you can probably tell from my notes, IMHO Setup #1, that has a slightly better soundcard than you currently own and has a top-of-the-line headphone amp, still
does not sound as good to me as the Essence STX does straight out of it's built-in headphone amp. The two setups were quite close, but listening straight out of the STX sounded to me like it was a minor but easily-recognizable
improvement over setup #1 (The HDAV + Opera).
However, it's important to note that like Shahrose said the strength of the STX lies in its DAC
section, not it's amp. It's DAC section is what allows it to sound better than setup #1 did. I say this because its built-in amp, although very good sounding in its own right, cannot compare to the sound quality I get when going from STX -> RCA outs -> Corda Opera.
So, you can probably guess what my advice is: Sell the D2X, buy an Essence STX, and when you feel like upgrading again later on down the road then
buy an external amp (keeping the STX as a source/DAC).