When I was looking for an amp / dac combo the one's I heard about a lot were the Schiit models, and the JDS labds O2+ODAC combo. I had the Schiit modi / Magni stack for a while, it was good.
I looked at the M50's when I was looking for headphones in that price range, what I ended up getting was the Beyerdynamic DT770 pro 80. They are pretty bass heavy, very comfortable and well built.