If it were just the comparison between different speaker amps and how they drive the HE-500 then ok, I could live with that and it's much appreciated. But I'm having a REALLY hard time believing the bold assertion that any reasonably powerful headphone amp such as the (small though it is) O2 couldn't drive the HE-500 well.
The cans are rated with a sensitivity of 89 dB by the manufacturer. Compare with the HE-6's 83.5 which makes a huge difference, the scale being logarithmic. Tyll's measurements specify 0.310 Vrms to reach a 90 dB SPL. The O2 outputs 0.630 mW into 33 Ohm (HE-500 is 38 Ohm), that works out to about 4.5 V (limited to avoid damaging low impedance cans) or about 1.5 Vrms. Don't have time to go into detail but look at the specs and do the math...it'll drive 'em with room to spare. I'm confidently asserting this not only because I trust Tyll, the designer of the O2 and calculations but also because I happen to own the epiphany acoustics O2 dac/amp combo, have an HE-500 on loan and actually own a HE-6 (for which the O2 indeed lacks power).
The HE-500 are driven just fine, even without extra gain. Volume rarely goes past 11 'o clock and I listen to classical which has a huge dynamic range. Some of the pieces I listen to I've heard multiple times live so I have a pretty good idea of how they should sound and I do recognize clipping and failing bass. The epiphany acoustics O2/DAC is my setup at the office, at home I drive the cans with a Violectric V800 DAC, coupled balanced to a V200 amp and that setup outputs 2000 mW at 7Vrms into a 32 Ohm load. Also sounds fine of course, matter of fact I use this to drive the HE-6. But just to say that this is a comparison point and I can confidently say that even just going by ear, the O2 also drives the HE-500 well. Also, as someone else remarked, Woo audio showcase the HE-500 with one of their amps. I'm not into tubes but I credit the guys at Woo audio with not being cretins....