with my limited testing methods(volume matched and a switch but no blind testing), they're pretty darn hard to tell apart. I won't pretend like they would sound the same to everybody on any headphone, but I doubt I could abx them successfully with most "portable" uses.
of course when using a very sensitive IEM, I can tell the O2 from the leckerton sometimes thanks to some channel imbalance in my O2 at very low volume setting.
and for hard to drive headphones, the O2 added power(significantly more) can also become an audible fact. depending on what you plan to use, you may want to check the power output and the gain values. some were bummed out, but the settings are allowing for a better IEM experience and volume setting. no point using a 1W amp and then kill it down to 8mW with a lousy volume control if from the start it was to use with IEMs. different amps for different purpose IMO.
but with average sensitivity portable headphones, both sound neutral, both have reasonably low impedance so they don't mess around too much with IEM's signatures, both are the most hissless portable devices I had in my hands. the reason I still have both. but again it really matter mostly on sensitive IEMs.
can't talk about the DAC section, to me they're all so close that I just don't bother wondering which is best. just that the 6mkII can survive longer with one charge when using the internal DAC I think, so I would have bought it instead if I had planned to mainly use the internal DAC. the amp part is what I use mostly, I only really plug the DAC into a tablet when I'm on a train or on a plane and it's to watch movies anyway, so my feedback on DACs won't help anybody ^_^.
