Is there a big difference in sound quality between HD 600 and 650?

As far as I know (but this is not certainly correct), both use the same drivers, and are built similarly, so there is probably not a really "big" difference. The acoustic design of the HD650 is more refined and allows for better bass extension and a more even frequency response, and judging from graphs it is possible that better matched left/right drivers are put into the more expensive 650 as well. The 650 has an overall darker treble, but it is still well extended, just slowly rolls off. Both are about equally easy or hard to drive, and technically the E9 is adequate for them.

They definitely use different drivers.

Not only that, they were engineered for two different purposes.  The HD600 was for critical listening, suitable for even professional use.  Sennheiser specifically said when they engineered the HD650, they found audiophiles were more interested in a non-fatiguing sound, something they can enjoy music with, rather than analyzing or critical listening.  That's what the HD650s are, they are among the best headphones for enjoying music with a non-fatiguing sound.  This is also why the HD600s were not discontinued, they are different and serve a different purpose.

Quote:

Originally Posted by mbd2884 

Not only that, they were engineered for two different purposes.  The HD600 was for critical listening, suitable for even professional use.  Sennheiser specifically said when they engineered the HD650, they found audiophiles were more interested in a non-fatiguing sound, something they can enjoy music with, rather than analyzing or critical listening.  That's what the HD650s are, they are among the best headphones for enjoying music with a non-fatiguing sound.  This is also why the HD600s were not discontinued, they are different and serve a different purpose.

Source?

Quote:

Originally Posted by vvanrij 

Source?

Sennheiser.  I gave you the source... WTF.

Sennheiser also said that the HD598 is actually much better than the HD800. The HD800 was specifically tuned for people with extremely large heads.

You can immediately spot the differences, but they're similar enough that most people can tell that they're from the same family. What I mean is, if you have an HD650, you probably won't need to own an HD650 or vice versa, since they produce the same sound sig, only with different emphasis.