Pros: audio & build quality
Cons: Portability & battery life?
Over a period of time I bought a FiiO E5, a CMoy and a FiiO E11, which all raised the soundbar. I bought the O2 because of NwAvGuy's writing.
The amp turned up one and a half month late and initially without an AC-AC power supply. But thanks to good communication with Epiphany's Oliver this was not such a very big deal.
The O2 drives my Audio-Technica ATH-M50 easily. My iPod touch V4 32GB is connected to the O2 with a simple FiiO line out dock L9 connector. Expensive cables are no longer necessary, I assume because of the high input impedance of the O2. I have attached the iPod to the O2 with 2 pieces of sticky double sided foam tape. I have put an old belt on an old leather bag to carry the two.
You notice immediately that the O2 is totally silent when music is not played.But when it plays... I have never heard bass, mid range, treble, sound stage and instrument separation of this quality.
Thanx NwAvGuy and Epiphany!!!