I can PM you with a link if you like, but there is a thread elsewhere
where purrin posted comparative measurements using impedance adapters which showed how the hd800 FR curve changes as output impedance goes up. The bass develops quite a hump.
Thanks for the link you posted! I've read a couple of other interesting benchmark articles on output impedance (eg how they advocate against push pull circuits because output impedance doubles). Along the same lines, I read all of nwavguy's proselytizing about 0 ohm output impedance back in the day. I agree that this school of design seems to make a lot of sense, but most headfiers don't seem to find measurement first headamps like the o2 or benchmark dac1 headamp to be ideal pairings with hd800s. (I love my agdr modded o2 with my ADs though...)
I'd love to better understand all this. Senns decision to run with a slightly high output impedance on their own amps seems consistent with the theory that 0 ohms doesn't yield ideal FR or damping with hd800s though. Thoughts?