Rensek
Headphoneus Supremus
You can buy one and if it's not enough, get a Magni 3 and use the Fulla as a DAC. Etc ...
I was advised by Schiit that fulla 2 might be a bit underpowered for the hd6xx. It's why I ordered a Magni 3. Fulla 2 is only rated for 40mw at 300 ohm. My phone and my iPad, both power the hd6xx, but they don't push it very loud, and the sound isn't nearly as full as what the Magni brings to the table. I use both phone and iPad as sources into my Magni. Mimby incoming. My 16 cents on this.