These are, imo, the best Sennheiser headphones. Among the many headphones I've tried, the only ones that feel really neutral and natural. A perfectly balanced sound, smooth yet very detailed. Bass has very good extension  and awesome overall tone and impact. Mids are magical, dead neutral and flat and you really feel this on voices and strings. Highs are very extended and smooth and there's plenty of detail. An awesome allrounder. Best value headphones out there, with X2 as a close second. Sounstage is very good but not on par in tridimensionality...

I own the desktop version of the O2. What to say? It is just perfect in every sense. No coloration , no noise, great power, best tecnicalities out there. BEST VALUE EVER. Endgame amp.

Very, very, very good headphones, Kudos to Philips for making such a stunning performer. Neutral headphones, they still mantain a very high fun factor and high trasparency. Very detailed yet very musical too. A true masterpiece. Bass is extended, creamy, textured, detailed and with good presence. Mids are perfection. Highs are smooth yet airy and sparkly, holographical. Not laid back in any way, yet smooth. Very very easy to drive, even from a smartphone. Very high trasparency and wide soundstage. They only lack the ultra uber high end tecnicalities...

All in all I would say a pretty neutral sound with very good extension on either sides and very very good soundstage. Very trasparent. Sometimes still a bit hard on the ears and sibilant, definitely not as neutral as HD600. Pretty balanced sound signature but with a treble peak. Sound is very good and clear but not good at all for the price. Bass has impact and extension, mids are very trasparent and realistic, higs have an annoying peak but otherwise are extended and balanced. Detail retrieval is very high but because of a peak in the treble it...