Pros: Very good for the price, easily upgradeable sound, runs on power directly from PSU
Cons: Has problems with low impedance phones, a bit tricky mounting discreet Op-Amps
Note: I'm from Sweden so if the wording or sentence structure is a bit off, I blame the Swedish educational system!!
The ASUS Xonar Essence STX is a two channel soundcard with a built in amplifier and it is a great entry into hi-fi and head-fi, it has a good set of features and handles most games very well, at least with the newer drivers. The card has a good DAC-chip that leads into two I/V stage Op-Amps and then on to either the headphone amplifier chip, called a Hi-Z amplifier, or to the buffer stage Op-Amp. There is also a decent mic/line in for recording. The card takes its power directly from the PSU and therefore gets a very clean and strong power supply, at least if the PSU is good. The software has pretty good features and the interface is pretty self-explanatory for the most part.
After getting this card I started reading the already massive thread started by ROBSCIX on this site, and soon enough I wanted to try to see what this card really could do. The sound with original chip Op-Amps was somewhat on the warm side and lacks a little something in detail but it is a good sound non the less. After reading 70+ pages of the gargantuan thread I came to the decision that the greatest improvement would come from upgrading to discreet Op-Amps from Audio-GD. After consulting some reviews and exchanging a few PMs with ROBSCIX I decided on OPA Earth for I/V and OPA Sun for buffer. This improved the sound through my loudspeakers with better positioning, better bass quality and clearer transients. The sound through the headphone-out became slightly more neutral, and the level of detail improved but not to the same extent as through the line-out.
The Headphone Amp
I do not have much experience with headphones, and my purchase of the K701s was a bit poorly researched, I just read some very positive reviews and comparisons but I failed to realize that it needs a good amp to sound as it should. Now I've never heard the from anything other than my amplifiers head-out and through the STX, and in that comparison the sound-card wins hands down. From what I've read about the K701 my impressions with the STX as source echoes what most people say: big soundstage, crystal clear highs, great midrange and weak bass on some types of music, especially rock/pop. The volume level is no problem achieving, with Replay-Gain on in Foobar, regular listening volume is at 60% on the low gain setting.
I've tried some of my other headphones through the head-out on the front of my computer, and it is very difficult to use IEMs such as my JVC-FX500 as the volume has to be set to 2-4% and at that level the cable leading from the soundcard to the front picks up some interference from hard drives and fans. Amping my vintage 600 ohm AKG K140 is no problem at all, the soundcard drives them loud and clear on medium gain.
This is a very good DAC and a pretty good entry-level headphone amplifier that costs as much as either would if they were external. The fact that it is from a big company such as ASUS means that the cost is cut by their ability to buy in bulk.
I can certainly recommend this card as a great entry to the world of hi-fi if you are using the line out from your stationary computer or when building a new one. The possibility to tinker with Op-Amps should also interest those who can't leave well enough alone...