A schiit vali is only needed if you want to use a mic. You don't so the mixamp on it's own should be sufficient. It's only when you split the power between a mic and headphones does the mixamp not provide enough juice for AKG's, simply put your dial to 100% game and you should be fine.
FYI I'm Scottish and own a Schiit Vali but it's not my video lol. I only just got my Vali on Saturday (4 days ago).
The vali is Schiit's budget tube amp, they have a lot better ones but this little thing is amazing for £100 (I bought mine off ebay second hand and only 3 days old the original owner bought it and it didn't live up to their expectations they were used to using gear worth several thousand pounds obviously a £100 amp wouldn't match up to that, so he ordered a Asgard or Valhalla iirc).
If you don't use voice chat then just get a mixamp, if you do then you will need an additional amp.