I haven't personally heard it, but everything I read says it should sound about the same as the HPA2 output of a Benchmark DAC1, which makes sense because that's what the maker seems to have used as a "high end piece" in most of his comparisons and studies on his blog - I believe he would often directly compare the O2's stats to the DAC1's?
I'm eager to hear the O2, and if it does actually sound audibly indistinguishable from the DAC1 headphone output, then it's definitely a very tremendously clear, colorless, transparent, and neutral amplifier. The question would just be, how strong is it and can it deliver enough power to make all of the harder headphones sing, and I suppose if you don't like colorless amplifiers, you'll find the O2 to be just as 'boring/clinical/sterile' as the DAC1. Many people here dislike the DAC1 and find it unlistenable (I am not in agreement with that camp).
As far as being a straight-flat, clean amplifier, I'm not sure what exactly noticeably and definitively bests something like the O2 in those aspects alone - the stats I've seen suggest that the O2 produces sound purely, "beyond human range of hearing flatness and clarity." Our audio technology is pretty advanced; and the O2 proves that it is not overly expensive anymore to make an incredibly clear, simple and precise amplifier.
Pure sterility and flatness doesn't necessarily make for the greatest sounding amplifier, though. Something "warmer" or what-have-you could be the preferred amplifier of the vast majority of discerning audiophiles. Plenty of people prefer something "artfully imperfect," sound slightly twisted to their tastes, over something "boringly pure." Or who knows, maybe the secret to the best-sounding midrange is actually to carefully put some peaks and valleys somewhere in the frequency response chart of the amplifier, and a "perfectly flat" amp just won't sound as clear as one that's a bit different. Human ears are funny things. Is a ruler-flat amplifier inarguably the best approach?
And don't underestimate the value of appearance and design. I'd be much more proud of a finely crafted tube amplifier on my desk, a clever and rare piece of equipment, than I would be of a simple and (in my opinion) very common and even mundane O2. There's a reason why the expensive speakers have polished, exotic wood cabinets, and it probably has little to do with the material's effect on resonance. It's because awesome-looking, well-made stuff is very attractive on so many levels. I haven't seen an O2 design yet that tickles my fancies the way some of the TOTL stuff does.
Edited by Timestretch - 4/4/12 at 7:49pm