Ok... I've got my O2mk1/srm-717 setup running for about 45-50 minutes now and I'm listening to my 2nd album as I'm typing.
So here's my 'super-early' first impression, based on my current understanding.
1) As expected, the O2 has a 'laid-back' and 'relaxed' sound to it from this particular Stax amplifier
2) Not 'that' dark sounding at all --> a 'brighter' HD650-ish sound
3) Even though it's no Edition 9, the dynamics is 'pretty good'! I'm pleased and quite surprised. I'm sure a beefy tube amp will transform this into a beast.
4) There's a noticeable treble roll-off. However I know this is partially due to how I've gotten used to my 3 dynamic headphones --> they all have pronounced and sparkly highs
5) Midrange. WOW! I guess this is one of the best attribute of the O2 I can sense as of yet.
6) Presentation. Alright, I'm not sure how to put this to words but there's something completely 'different' about the music presentation; nothing I've experienced on the other 3 headphones I have on hand. It's not the mid-sized sound stage, it's more of the instrument 'placement'. Maybe 'precise' is the best word for it. The sound of the base guitars, cymbals, or vocals from the pianist is 'there'. For comparison's sake it's not like the Jade driven by Ray's amp (Chinucifi) where there's a really noticeable instrument 'separation' (not placement) where I can pin-point where things are going on inside my head-stage or the openness and 'blending effect' of the HE90. I guess it's somewhere in the middle of both. Again, all I can say it's a sort of presentation I'm not familiar with.
With that said, for me, the O2mk1 is a very very comfortable headphone. I like the soft ear pads and the comfy head band (not the arcing metal band lol). I also like how the ear pad mechanism can rotate like that; it's pretty cool. I can see the O2 being a good companion for LooooOOOng listening sessions, like the SA5000. I think with my current setup, the O2 is clearly the best all-around headphone: which is one of the reasons why I bought it in the first place, instead of the L3000. I don't think it'll outlast the R10 in classical genre/vocal based tunes, nor the Ed9 with rock genre, nor the SA5000's speed and 'rendering' capabilities in gaming.
Clearly, like the Jade and HE90, I need to spend a lot more time listening to understand it's attribute. It's a fine headphone and I'm not disappointed by it. I'm happy it's by my side come Christmas time and New Year