So momomo did you actually mange to fix it? A week a go i bought a used asus xonar st and i seam to have the same problem. Before i bought it the guy who sold it to me he tested the card with Sennheiser HD 558 which sounded clear. Once i hooked my amp and speakers i notes the sound was muddy and lifeless just like you experienced. Here is my setup: amp yamaha a-s201 and speakers heco victa prime 302. Oh and i almost forgot i did tried with some cheap speakers like 10$ and with those it sounded clear. I would also want to say that i have an usb dac(nuforce udac-3) which sound good on my main setup.Any thoughts what is going on here?