what formats are you listening to now? Spotify streaming and mp3s are fine with simpler gear (FiiO E10k). The medium gain MD O2 amp and JL labs ODAC is good. If you want to do more with High Res Audio look for a DAC that can handle DSD formats. Topping D10 is filling in that role nicely for me
My next amp/DAC purchase to feed my HD6xx is the MD CTH, probably with the SDAC. It’s a well reviewed pairing and the price is in the same neighborhood as the HD6xx. I dont have it yet, so can’t recommend. I’m curious about balanced amplification and might pick up a MD 789 or Schitt Jotunheim somewhere down the road.