It's largely a matter of perspective: If certain headphones don't pair well with a neutral, low output impedance amp, I think NwAvGuy would point out that it's not the amp's fault. I would agree here, since designing around such an amp is really the only way to get a stable reference point for headphone performance: The concept of amp neutrality is objectively well-defined, but coloration spans a huge multidimensional spectrum. Similarly, 0 ohm output impedance is well-defined, whereas banking on pretty much any other output impedance is totally arbitrary. For the sake of standardization and keeping consumers from going crazy and broke, headphone designers should really be designing around 0 ohm output impedance amps. If certain headphones are depending on a high output impedance amp to roll off certain frequencies that the headphones go overboard on, I'd personally consider that a design flaw in the headphones.
In a perfect world, all headphones would be designed around neutral, low output impedance amps, but this is the real world...and sometimes maybe they aren't. Even if they are, headphones are impossible to make perfect with our current technology, so even some headphones technically designed around low output impedance amps might have flaws that some higher output impedance amps could alleviate by happenstance. (Or maybe you just prefer muddy bass?
) At that point, you pretty much have to search for a Goldilocks zone...but what if a particular amp DOES compensate perfectly for a headphone's flaws? In that case, the amp and headphone combination becomes much more than the sum of its parts. Independently, I'd consider both flawed...but together, they could be a perfectly viable or formidable choice for great audio.
Ultimately, no amp will be the best choice for all headphones. However, I think a powerful, objectively well-measuring, neutral, low output impedance amp is inherently more versatile than colored amps: It may not compensate for any sonic flaws in the headphones, but it also won't introduce any of its own that will exacerbate them or otherwise make things worse. Neutral amps may not sound great with ALL headphones, and saying they do would be an overstatement...but they're a good middle ground between warm and bright, and they're really the only stationary targets for designing headphones around. The same applies to low output impedance, since the effect of high output impedance is an arbitrary sort of frequency response rolloff, which would count as a source of coloration.
No headphones are perfect, and there may be a particular amp that matches a particular pair of headphones better than the O2 will ever match anything. Nevertheless, I want the versatility to try (and judge) a bunch of cans without worrying about amp coloration as a variable or trying n*m combinations of headphones and amps, so the O2 is perfect for me. If I need to tweak, I'll go for an EQ or DSP. (It is interesting though that you could add a switched output resistor to it. That would definitely increase the versatility aspect further.)
However, if you're set on a particular flagship headphone, you may get more mileage finding the perfect amp for that particular headphone model. If you're a serious headphone hobbyist (deep pockets
), you may get more mileage just playing with amp options too.
