Well it would be boring if every headphone sounded the same. I personally love the 650/6xx because of its tonality, you can listen to music the whole day without fatigue and everything sounds butter smooth. I actually think they’re really unique in that way and no other headphone does this as well as them.

But I found that the best of both worlds for me is the HD800. It has that neutral Sennheiser sound, they opened up the soundstage big time and brought the treble up. Yet it still somehow reminds me of the 600/650, they even managed to improve the superb comfort and take it to another level. Maybe this could be an interesting choice for you, second hand they’re only like 600 bucks. Still pretty steep but they are such a great value imo.