hmmmm. im enjoying the conversation going on here. im afraid i dont have enough technical knowledge or audiophile experience to contribute much, but i would like to add something nonetheless.
the creator of the O2 did say that the amp could compete with any amp under 500$. so, continuing the discussion on whether or not you can have a "better" amp for more money, going on the description of the creator, would you say that >500$ amps exist that do sound "more transparent"? excluding features, looks, build quality and things like that, just sound.
No because while there are >$500 amps that have either more power (which one or two bizzare headphones benefit by) or even lower noise/distortion performance, the latter is effectively irrelevant because the O2/ODAC performance in those terms is already at inaudible levels and will make no difference to experience let alone pleasure. So while >$500 amps and dacs might have better performance, it's academic really. The O2 has this even at high gain; so clipping the resistors to configure the minimum gain to 1x shouldn't be done for the sake of audio performance but rather because at the default 2.5x it is already too much gain for many headphones (since the volume knob has left-right audio imbalance when turned down too much) unless you turn down the source.
And logically, as you've probably guessed, it's impossible to be more transparent. I suppose one could say nearly transparent and even more nearly transparent, but since the O2 is said to be (and independent tests confirm it) actually transparent, then you can't go anywhere from there even if you do spend >$500.
So unless you need some kind of fancy feature that the O2 doesn't have, or have the HE-6 or AKG100, then the O2/ODAC is all you really need. Having said that, the magni/modi offering from Schiit is a designed-by-measurements transparent combo, and cheaper and smaller (and prettier). Being portable and having less power, there may be more headphones that combo can't power, but for many they are worth considering. If you just need an portable amp then another transparent by-measurements design is the JDS C5, which looks good indeed.