adydula (& agdr),
After having this setup running a little longer (and having run all of my headphones from it), I have the following further impressions (subject to the limitation that I do not have a 'stock' O2 on hand to compare to).
It seems like output impedance has a lot to do with what I hear through my IEMs. You might not think that there might be that much of a difference between the stock unit's already quite respectable 0.54Ω output impedance and the near-zero output impedance the booster board gives you if you jumper or short the 1Ω resistors at the output, but apparently there is, at least with my IEMs, which run from 16-36Ω (and probably low-impedance fullsize cans too, just don't have any around to try). Detail is definitely 'cleaner' overall. Bass is tighter, and leaner in the mid-to-upper bass (maybe even seems a little
too lean at times with my already lean Ety hf's) but still reaches into the really deep territory with power and 'punch'. Probably more
accurate (my SE535's shift from 'warmish' to a little more neutral) but, like any change in an audio setup, sometimes changes can take some time to get used to.
Typical volume settings with the IEMs (and the 0.67X input gain as modified by agdr run around 9 o'clock with the very sensitive Shures, around 10 with the Etys, and 10-11 with the Apples. Generally 10-11 on the 300Ω Senns, with the gain on High.
The high impedance cans, to repeat myself... I hear a little deeper into the mix and instruments separate a little better, but it's subtle. If your favored cans include any under 100Ω, or the other worthwhile improvements like the relays appeal to you, I'd say run don't walk to your nearest agdr dealer
