"the O2 compare quite linearly" Meaning they sound pretty much the same?
I played a little bit with the high/low settings, and with my bosshifi b8, it does appear that the high setting adds a subtle bit of bass. Since low impedance is less than 25 ohms, and the b8s are 16 ohms, the low setting would seem more adequate, and yet the high setting seems to provide a slight bass boost... Does this make sense or is this placebo? I think it was similar with the AKG 550, 32 Ohm, but I don't quite remember as it was a couple of days ago and the differences were kinda minute if there at all... But for comparison, the Aune T1's amp section, for some reason, adds quite a bit more bass, which makes me wonder if it could also be an impedance 'issue'?
Also now makes me wonder if using the smaller headphone jack on the amp also result in a sound difference? I remember it being less loud, but I haven't really tried to volume match it and listen for actual sound differences...