I recently bought the atom DAC+ and AMP+ by JDS LABS for my hd600. Can only recommend it. They are available in 2022, which is not the case for most things in this list, and they are only around 220€. The quality is better than the fiio k5 pro in my opinion which is about the same price but since the combo has less warmth than it it depends on the listener. In some cases there is no need for a DAC/AMP so think about it and read about it before you waste money if you are new.
Edit: typos
Honestly a basic Magni/modi Schiit stack is all you need unless you plan on purchasing a better headset in the future. Either that or an SMSL M6. Both deliver good performance, especially in their price bracket.
The Massdrop Grace X has been phenomenal pairing: tons of power and tight extended bass, yet musical and transparent with a wide immersive sound stage.
The Parasound Zdac v.2 DAC and Headphone Amp is a workhorse with an incredible build quality. It works well with the FiiO 3.5mm coax outputs and optional cable and with the Cayin N3 optional USB-C to RCA cable (though the USB-C port is also the charging port). (A rack-mount package is available for the Zdac.) I also own the SONY PHA-1A for portable use.