Didn't have that high expectations on this amp but it surely surprised me with the open and accurate sound it produced. I use an Audioquest Dragonfly v1.2 DAC with this and they sound really good together.
My main headphone amp is the Little Dot mk II tube amp which has a lovely warm sound that just flows through the songs. The O2 is a strong rival to the LD though with it's accurate but easy sound. A lovely pair specially to the Sennheiser HD6XX headphones.
Definitely recommend getting it! But please fix the damn plug to an EU version (or atleast provide the possibility to get one).
EDIT (03/2019):
The more I've used this amp the more I love it. This has almost completely replaced the LDmkII as my daily driver. And now I actually prefer it paired with the HifiMAN HE-400 so I can also highly recommend the Massdrop x HIFIMAN HE4XX + O2 amp combo.
nortelcI have the Standard Gain (2.5x Low, 6.5x High) -version.
Should probably take it in active use again. Been mainly using the CTH for a good while now.