Schiit Magni2 or JdsLabs Objective O2 desktop edition are two relatively cheap amp that are perfect companions to your HD6xx. Depending on the version and options chosen, they each go for between 100$-150$. Both amps have also their corresponding DAC whch may be bought later: OL DAC or Modi2, again going between 100$-150$ each depending on the version and options chosen. so in either case, the stack solution would cost between 200-300$ depending which versions and options you choose. The Schiit Jot is more expensive: 400$ for the amp alone or 500$ with the DAC add-on. All of these solid state solutions are safe bets and are great companions to the HD6xx, providing more than enough clean power with close to very neutral sound caracter. You can't go wrong with any of these.