The Asus STX have plenty of power, so unless
@Nobunaga use the front headers running thru the PC, I'm a bit baffled.
To be fair I've done a similar "upgrade" from using the front ports of an Asus DGX ($50 soundcard) to an external amp/dac. Mostly for features and convenience. To me (trying to do some blind testing), I even preferred the sound from my old Creative DDTS and the analogue out from my CCA, compared to the DGX. But then again the DGX is a low end card, with really cheap components onboard compared to a STX.
I don't really like the headphone amp on the DGX, not enough power despite being a card with a dedicated amp.