Are you looking to be portable or stationary at your desktop? The choice of an O2 makes me think you're not going to be carrying it around.
I believe the Objective2 and the equivalent Schiit stack are some of the top contenders for that price range. SMSL and FIIO are some other brands knocking around for amp/dacs as well.
Oddly one of the Behringer USB Audio Interface devices (
UMC204HD) reviewed as being better than the Schiit Modi 2 so that is another thing to be considered if you don't have headphones that need that much power to drive. I personally use a UMC404HD as I wanted the dedicated wall plugin over USB powered only, and I use a discrete microphone since I've gotten tired of shitty headsets. Probably not going to fit your usage though if you're just plugging headphones in to it though.
Paging
Brahma
to give some ideas too.