Mark_MThe HD600 has it's own distinct sound, and there are times when it's the perfect fit for the music I'm in the mood for. I have all the headphones in the series (minus the original HD580 and 580J), and I use them all in regular rotation. The 600s can regularly be purchased for a very reasonable price, and if you want to add to your collection it's a worthwhile purchase; at least in my opinion.
Mark_MNo it's not. Save those money and put them towards a different headphone.. If you like sennheiser, why not hd800s? They're simular enough that it doesn't make sense to own both. IMO it's so easy to buy every mid-range headphone since they feel so cheap in terms of $$$, but then you end up with $3000 in medium headphones, when you could have a more mixed collection.
Mark_MYes, I have them all including the HD660s. If you got the dough give it a go, you can't go wrong with HD600s. I prefer them over my 650's, 58X's, HE4XX's, and 7XX's for that late night quiet room listening session. You truly hear every little detail with HD600's and they sound amazing tubed, a decent tube amp is a must if you're going to own that many pairs of the Sennheiser lineup a lineup that loves tubes.
JustinBotrosIn all honesty the 660s are better than the 6XX across the board especially for gaming and ease of use portable. That being said they aren't twice as good or anything probably 10% better in all aspects. The 4XX have a slightly wider soundstage than the 660s and a tighter planar bass but have significantly less detail and imaging overall. 660s has better minds than the 6XX but the 6XX I better mid range than the 4xx. If I had to rate them in order for music 1.660s 2.6XX 3.4XX for games I'd say 1.660s 2.4XX 3.6XX. Things to watch out for with the 4XX is that they get warm in the ears of your room is above 78 degrees.