I have to correct myself as I just had the chance to compare the O2 Mk1 on a test CD with natural stereo recordings. It is a really nice listen but the way it was configured at that particular test it did not sound like a genuinely accurate headphone.
The HD600 seems to strike a good balance, for instance I would say the O2 Mk1 is too warm while the HD800 is too cold. I would still take the O2 Mk1 for music listening though for its warm and seductive, non fatiguing sound.
Also some people use extensive EQ to match their own HRTF with the headphone they use to achieve a more accurate sound. As I read the O2 Mk1 transducer is fairly flat (as most electrostatic headphone transducers) but the way the headphone is designed it will be heavily affected by the person's own HRTF (one reason why Stax calls their headphones as 'earspeakers') which will result in a more coloured sound, that also explains why it is so sensitive to the fit.
With an accurate IEM that correction might not be needed.
I am inclined to try the SR-4070 though as it might be more suited to this task while still carrying all the technical abilities electrostatics are famous for but the huge size and weight are quite worrying.
Edited by padam - 12/2/11 at 11:01am