Need help on the following questions. Which one is better as an "dac"? Which one has a better built-in amplifier? Which one (or neither one?) is good for playing with DT990 600ohm, HD600, and k702 without an external amplifier?

I actually brought these accidentally, and no sooner than I paid for this that I was noticed by my friend that I need a really hi-end amplifier with it. So, I am here asking for recommandation. I even haven't opened the box incase I have to sell it. I wish I could have do more research before...