For the HD6xx if you don't care about other headphones (high impedance output) the Aune is good, otherwise the RHead was also rated favorably:
https://www.audiosciencereview.com/...aune-x7s-and-arcam-rhead-headphone-amps.4270/
You've missed the drop for the THX AAA on massdrop which is surely their best amp, but monoprice has a THX amp releasing soon, too which might be quite good, that one is a dac/amp combo though. Neither THX has been reviewed by amir at this point though.
I would not bother with the Liquid Carbon or the CTH. The massdrop O2 is probably the best budget thing out there afaik.
Previously I was using the o2-odac combo ( done by a local DIYer ) but I sold it off because of some bass distortion and it's cranked UP. Not sure if I want to re-visit it again.
As I need <1 Output Impedance. I guess only o2 and rHead.