With the O2, there is no "recommended setting for headphones list" for which headphones for which gain setting.
For my headphones in the 32-Ohm to 80-Ohm range, I leave the gain set to low
For my 250-Ohm and 600-Ohm headphones I leave the gain set to high
The 80-Ohm DT770s, will work fine with either setting.
62-Ohm AKG 7XX series (which i do not own) like a lot of juice, so using the high gain setting might be better for them
It might be a little better to just leave the O2 set to the low gain setting, unless you get hard to drive (or high Ohm) headphones.
Alright, think i got it :) thanks once again for your help. Will leave "gain" off.
Thank you so much for your help PurpleAngel