Let me start off by stating that I own the Sennheiser 600's too.And this headphone isn't that.The 600 is a very Audiophile quality headphone.This version is more a 50% Audiophile 50% enjoyment headphone!An Audiophile headphone is flat and VERY detailed! 3D, pretty much as close as possible to actually being there in the studio. At least that's what an Audiophile looks for.The 660S isn't flat. Pretty detailed but the bass is bumped up a notch or two.This headphone is more enjoyable for the "Normal" person. Like I said, it's very detailed. You will hear sounds from your previous favorite tracks that you've never heard on other cheaper headphone/Earbuds. Ticks, taps clicks, clings and so on, lol!Pretty much your music is now High Quality!And you will be able to tell with these!The 660S are open back. Meaning you can easily hear other sounds around you. Someone speaking to you or a television in the background. It'll ruin your music experience. You need to be in a quiet room or away from others. If you mind?They aren't closed such as a traditional headphone that we've all pretty much are accustomed to. The open back assist with the awesome sound! Giving it a more open airy sound. Not a HUGE sound stage so don't get me wrong. Not like an 800 Sennheiser! LolThe 660S headphones that go over/around your ear are very comfortable! I have a big head and they feel light and comfy!The clamp around the ears isn't bad for me. I can move my head aggressively and they don't slide off. But they aren't clamped on with a lot of pressure either. They're perfect!The cord that it comes with is pretty long! Not many are fans of. If you wanna just connect to your phone and move around your house jamming out? These aren't for ya, lolAt least the cord that it comes with isn't. This headphone follows after the 600's. Which are meant to be experienced at home. Connected to a high def device. In which you are given plenty of cord so you can sit down with comfort! Unlike other headphones where you move around and you end up yanking the headphone accidently or however. These are not mobile Headphones. But these do not require Hi-Def. Devices! That's the cool thing about these compared to the 600's.The 600's require a lot of power and clean power and a VERY good Dac for ultimate Nirvana sound!Pretty much you need to put in a lot of money towards your investment with the 600's!These 660S's you don't. An Audiophile on a budget can easily enjoy these!A simple headphone connection and you can enjoy these without wondering what else can you attach to obtain it's optimum sound!Of coarse, it is 50% Audiophile grade so use the highest quality connections you can with these! The better and cleaner the better the 660S will perform!But don't stress!They are 50% relax and enjoy type of headphone! And in my opinion, they are AWESOME! 😉👍👍👍Plus with some extras, true Audiophile grade headphones provide!If you're looking for a Pro, flat Audiophile sound? These will not satisfy.The 600's will.If you're someone who enjoys good quality music but don't want that really flat tuning. And don't or cant really obtain expensive devices to enjoy them? These are for you.They are awesome!I haven't used the Balanced cord on them. So I can't write my opinion on that.Sennheiser, keep it up, you guys are AWESOME!For those that haven't tried out a Sennheiser headphone yet? Do it. Give them a try!Sennheiser doesn't flake out on premium sound! They are ALL about it!Plus, with Amazon you can always return them if you aren't a fan of them for any reason!30 days I believe?Enjoy!I know I am!(As we speak!)