Questions on astro mixamp

Just recently started to get more into audio stuff. Just have a few questions. How good is the astro mix amp? I also have the maximus VI formula motherboard

The onboard audio says it can push 600ohm headphones which I plan on getting. Since I have the mixamp plugged in with an optical cable is my onboard audio being bypassed and not being used at all? 

Optical bypasses all processing on in your computer. It sends it all to the Mixamp in that situation. I don't really know how good of an amp the mixamp is because it is mostly a console gamer's tool. It is generally used for its connectivity, not for its audio properties. If it sounds good to you though, who cares?