For me this was the first set of headphones from the 600 series, and I have no regrets. I have several 500 series and they sound great don't get me wrong, however there's a different kind of change when switching to the HD 660s. particularly for me orchestral tracks sounded more vibrant and cleaner, what was once drowned out by other instruments is now very clear. I in particular enjoy listening to movie soundtracks. Movies like Krull are an excellent example of the power of the HD 660s because of how lush and dense the soundtrack can be. Particularly tracks like "Ride of the Firemares" and "Battle on the Prarapets" where there's a large orchestra performing a very vibrant and active piece, things like the triangle are no longer drowned out by the horns and strings, it's very clear. Things like the the string instruments now have a clarity to them that feels even more realistic than before. it's really unreal the clarity you get. All that mixed with the fact that they're only 150 ohm with a higher sensitivity, makes it easier to drive than some of the other 600 series headphones. I personally I use a FiiO M11 with the HD 660s with the balanced output, and it sounds fantastic. I really can't recommend them enough or describe it in enough detail, it's something you only truly understand of you hear them for yourself.
