Thanks for all the input! Judging from the responses i guess the 250ohms dt880 would be a much better match for zero than the 600ohms...
I think i'll just drag my zero all the way to the store to test it again myself to confirm it after burning in the zero amp properly. And check out hd650/hd600 + zero while i'm there.
I wish burn in could be done faster =(
btw if i change opamps do i have to "burn them in" again seperately?

sorry for all the noob questions! And much thanks to everyone for this thread.