Which input should I use on my HD6's for the best sound quality?

All digital audio sources will ultimately need to be converted to analog to provide sound through any speaker system. Any optical/Bluetooth input will pass the digital signal on to the HD6's internal digital-to-analog converter (DAC), while the RCA/3.5mm inputs are already analog and have been converted by the built-in DAC in your source. So it's a question of which DAC sounds best to you. We would recommend testing each input separately to see which one you prefer!