In case you missed the giant facepalm...the entire 600ohm headphone amp expression does not make any sense.
One measure of amplifier performance is internal impedance, or called amplifier output impedance.
Amplifier Output Impedance
What the specification on the ASUS card tells you is the SNR when used WITH a 600ohm pair of headphones.
I use a pair of 600ohm DT880s with my Xonar STX, and there's no way in hell am I going to be satisfied with the way it sounds straight out of the card. It's not powerful enough. That's why I have a line-out from the STX to a Schiit Asgard for properly amplification.
Also regarding gain levels. As you go higher you will lose dynamic range..so keep it as low as you can