I own both K550's and 65's and can say that if you like the 550's you will absolutely love the 65's. They are actually quite similar sounding, but the 65's have wider soundstage and are even more comfortable. I also feel that the 65's are smoother and have better balance overall. The (semi)open structure of the 65's enable more transparent sound. If I had to choose I would take 65's any day, although I love my 550's (the best closed cans I've ever tested). Bass wise I would say that the 65's are pretty close to the 550's. I have also tested standard 702's vs. 65's briefly in the same amplifier and came to the conclusion the 65's are warmer/fuller sounding than the 702's (straight out of the box). This is, however, only my personal opinion but I'm pretty confident about my impressions. It is yet to be confirmed whether the fuller sound is coming from the new memory foam pads, from different tuning or from both. Some people say that by switching standard 702 pads with the memory foam pads you get 702's and 65's to sound very similar. In my FiiO E17 the 65's need about 20% more volume than the K550's to get similar strength of sound.