As a general matter, I can't wrap my mind around why we are discussing the O2 and HE6 in the same sentence - maybe we should assess its performance with K1000's or maybe a pair of magnepans while we're at it? The designer was totally candid about the fact that the O2 wasn't intended to drive outlier phones - which very few head-fiers own to begin with. Fostex and, to a lesser extent, Audeze orthos are certainly fair game for discussion. There is no question that the O2 drives the former very nicely, and we seem to have some divergent opinions as to the latter. Fair enough. The amp is also excellent with iem's at unity gain, and I like it with my dt880's but no so much my hd650's for whatever reason (although I should really try them again with the agdr booster in place).
Don't take that post too seriously, O2 is very good indeed but it has its limit. I think the guy who posted that comparison just doesn't have really good hearing ability. I haven't heard the B22 with my own gear and setup but O2 is still 1 step down compare to my PPAV2 and 2 step down compare to my friend's Dynalo, with me i can hear the different very clearly and the O2 is definitely not up to level of these two so i don't even care to compare it to B22.
These are valid subjective impressions, but I think it is worth noting that other veteran head-fiers (e.g. as quoted below) have arrived at opposite conclusions on these points. On the specs, the O2 trumps both of those amps in spite of the fact that the ppav2 measurements were made without a load. So the interesting question in my mind becomes why many of us ultimately prefer other amps (at least with particular phones) that don't meet the same measurements. I love my mainline, for example, and have often hypothesized that a certain amount of even order distortion is making up for the room effects we don't get with headphones, creating (for some of us at least) an illusion of music that sounds more like a live performance in an acoustically pleasing venue.
I also chuckled a bit at the comparison to the dynalo since just the other day, USG, a head-fi veteran since 2004, posted the following about his GS-1 over in the hd650 thread: