For a studio headamp the O2 is not the right choice. The OP needed up to 800 ohms capable, probably meaning he is replacing an amp for phones that he uses with a 680 tap which is common on old mixing boards. Its easy to infer he is using older low efficient high ohm studio headphones.
Having built an O2, I can tell you that it is not anywhere close to being sufficient (as example) for the emmensly popular AKG Sexttets (600 ohms), the OPer probably has another these or another AKG 240 variant, some of which had even worse efficiency than the AKG sextetts.
Why does the O2 or similiar OPA output headamp have issue with the old AKG studio headphones?: not enough voltage swing. Yea the Sextettes were advertised at 94db/mW but I can assure you this was marketing, probably measured with the mic placed right on the driver, they ain't 94dB/mW efficient. You can do all the math you want but real-life all we know is are max rated for 200mW, so you want a headamp that can deliver 11Volts RMS of clean power into the 600 ohm load, the O2 can only deliver 7Vrms. The jfet amp recommended early also doesn't have enough voltage for these cans, so this isn't a knock on the O2. It simply wasn't designs for the millions of popular yet obsolete studio headphones many still use.
In the DIY world of published designs I have only found three SE-out amplifiers that can power these old AKG's (sextetts, 240df, 340, etc) with reasonable low distortion: dynahi (PCB's are unobtainium), betta 22 (expensive and time consuming build), and the EHHA ( the tube distortion is kncoked out with heavy nfb, its really a discrete opamp like the b22.) All of these are large formfactors, and issue for the studio.
The OP is probably much better off buying an old Sansui integrated amp. There are reports that the violectric (lake people) amplifiers are working OK with these old studio headphones, they get around the issues with dip opamps and high voltage swings buy incorporating a TO-220/DDPak heatsink mountable buf634 in the opamp feedback loops. This way the ouput can handle high voltage swings/lowish current and low voltage swings high current, something near impossible to achieve with dip8 package opamps. But even these amps are on the edge of not having enough voltage. The O2 designer said himself that one is better off looking at other amps for low efficiency high ohm headphones (that are much more popular than people on head-fi can fathom
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The other possibility which the O2 designer is against is going balanced, with a separate dip opa based amp for each channel. I personally think this is the answer for opa based headamps and these popular studio headphones, the wire I believe has a bal-bal version but I don't know if it has 2x the voltage swing of the std design.
Hopefully we will see an small dip opa design with 4 channeled balanced operation for phones like these in the near future.
