The MDR-7506 or its professional counterpart (gold plated instead of silver plated plugs) have been the main stay of recording studios around the world. It has an extremely flat response curve. But some people find these flat response curve headphones too bright to their liking. This is my experience as well.
The Sennheiser HD600/HD650 has a slight midbass bump and a bit recessed trebble, making them sound more palatable to a lot of people, myself included. I prefer the HD600 over the HD650. The HD650 has an even more pronounced midbass bump compared to the HD600, making everything I hear from those headphones like I am hearing sound passing through thick maple syrup, not my cup of tea.
The MDR-7509 is an improved version of the venerable MDR-7506, I have never tried that model personally.
Having said all this, I currently own a pair of Sony MDR-SA5000. AMAZING bass response, the best I have ever heard (better than my old stereo setup). I like the Sonys for their fast response. This aspect of high end Sony headphones make the high end Sennheisers sound dull in comparison.