I'm a jazzpiano player, and so I'd say that I'm somewhat familiar with how instruments sound live, and the detail that the 660S delivers is really good.

Now I listen mostly to jazz and acoustic music. So I brought them back to the counter and left much poorer, but happier, clutching the 660S to my chest. Now I admit that the HE400i might be a bit more 'fun' on some of the tunes with a lot of bass, (and I've probably got buyers bias) but the price you'd end up paying on acoustic tunes was not worth it to my ears. So I decided to switch between the headphones and just listen shortly to each tune but no the HD 660S were just that more open and realistic compared to the HE400i. Now at this point I had been listing for an hour and thirty minutes to the 660S, and was thinking I didn't remember the sound of the first tunes properly. The soundstage was worse, and the instruments just didn't sound as good like they were wrapped in wool. I got the Sennheisers setup at the store, with some fancy equipment way out of my price league, and went to town being all critical and sh*t, writing things down on paper so I could compare, going through jazz, neo-soul, choir pieces, pop, orchestral, the whole shazam! I was enjoying myself immensely with the detail of the 660s (more on that later), but stil being critical and focusing on all the 'bad' things with the headphones. I ended up with choosing between the Hifiman HE400i and Sennheiser HD 660S, as the HE400i were on a big discount which put them in the same price braket. So my old AKG's were starting to show their age, and with christmas I decided it was time for an upgrade. Excuse my bad english second language and all that jazz!

Can i join you guys