I currently use a pair of Audio Technica (AT) ATH-AD 700. The old pink/gold coloured model. But it is getting a bit old. So I was looking for something new. My headphones are only used for gaming and YouTube. My setup is as following

PC-->opticat spdif-->Wadia 12--> headphones

Volume is adjustment is done via windows.

The heaphones I am looking at are the
1) Beyerdynamic DT 990 edition
2) Audio Technica ATH-700x
3) Sennheiser HD 560S

Bonus question 1, would something like a Schiit Magni be an upgrade?
Bonus question 2, would a 2nd hand DAC/headphone max 300 euro be better?