It definitely doesn´t have less bass than HD 600, but it doesn´t have that much more either. Perhaps a bit. The bass is mostly just better defined and faster. I´m not sure if there is more impact, but the impact is sounds different as the bass is just faster and better defined on the new transducer. I prefer the new one. That being said, I think open headphones are already pushing as much bass as they can, so increasing it (or to be more specific making it go deeper) substantially is probably technically impossible with a driver of this size. As for fuller and warmer I´d say the HD 600 is warmer than the HD 660 S. The faster crisper HD 660 S sound is closer to what I perceive as neutral. HD 600 has one big possible advantage (depends on taste) over the HD 660 S: it´s more forgiving/relaxing to listen to. Very brightly mastered low dynamic range music may sound better on the HD 600. I personally won´t be keeping my HD 600 though. In any case they sound quite different. As with any new headphone however I would suggest finding a retailer that lets you demo them with your own familiar music. There´s no substitute to hearing them yourself and then making your own choice based on A/B comparisons at the store.