While I love Asus, I have the DX (PCI version) and on my Asus motherboard, the static and popping would be kind of horrid at times, then go away. I tried many options to get the issues resolved - muting the mic, new drivers, etc., but nothing worked. I then opted for the DSX (PCi-express version, with a swappable op-amp module.) While the sound quality increase was a notch or two better, it still suffered, at times to the static and popping, too.
I was quite patient and methodical in work through the issues, and didn't get frustrated and give up quickly. The sound card had been fine with just driving the speakers built into our LCD monitor, but when I wanted to use headphones connected to it, that's when I started to discover all of the issues. I then did some more research. It seems like for external USB varieties, there are some good comparisons on the Internet between the USB-based portable sound cards. There is the Creative Labs x-Fi iGo controller, the Asus U3 USB portable device, and many, many others. In my case, I picked up the Asus U3 as well as the X-FI iGo unit. The X-FI iGo is still in the box, but I'm using the Asus U3 card at the moment with headphones, and I've alleviated what I believe to be all of my frustrations. When the music is not playing, there is a very, very slight noise in the background, but it is not present when listening at normal volumes. With the other slot-based cards, the noise would easily creep up and above the music listening levels.
I'm currently using FooBar with the wasAPI driver. I've found that the sound is pretty good. Actually, quite good compared to the PCI offerings and their issues. Granted, I'm sure there's something that's being held back a bit by the super-small USB card's footprint, but to my ears, it's doing pretty good. I can use the EQ sparingly to tune into my desired sound. Having a quiet card that ins't popping and emitting noises, is really a joy.
I would read reviews by people on the USB based sound cards and would often think, or wonder what all the fuss was. Or, I'd wonder if they were just too lazy to install an upgraded card on the motherboard. But, then I realized that the devices are designed for laptops, too, and their internal sound cards can leave a lot to be desired. So the option of using these cards on laptops is quite viable.
Now, back to your specific question. Personally, I'd look at the STX as you've proposed. The flavor with the Muses chip seems appealing. Plus, if it performs like my U3 "mini" Asus sound card, I'm sure it will provide you with many hours of enjoyment and not having to spend hours trying to resolve the static and popping with the potential that the situation might not be fully remedied, makes the "Audiophile-Grade" Asus STX desktop sound card quite attractive.
Best of luck to you.