It is an odd one really. I have been using various Sennheiser phones since the 1980s. I was put onto them by my favourite record shop. They used them to allow customers to have a listen to records in stock! They would not let the customer put the record on for themselves though!
Over the years I have tried out various types from recording studio types, Grado, Bayer, and and many others I cannot remember the name of.
I find the HD 600 phones to be "ideal" for me. They are the best Sennheiser phones I have tried. I have never tried anything that I could not afford though!
I am sure that everything in transducer design is a choice of compromises. For example I have been told that Stax make the least compromised headphones, but I would never pay the asking price, or even get second hand, as it is not so clear cut as to support when something goes wrong.
At least if the 600s go wrong, then I can afford to replace them if beyond repair. So having them gives no fear for anything going wrong that its beyond me being able to afford to put it right.
I think that is important. I had a work colleague, who ran a high spec Audi A4. Its Diesel Particulate Filter went wrong, or at least diagnosed as such by one garage who quoted four figures for the repair! He was terribly unhappy as he really could not afford that kind of repair. As it happens my friend is a second generation Czech, born in this country, and he asked me what to do. We got on well. I told him to take the car to a different garage, which then reported that there was nothing wrong with the DPF. But I also told him once he had a clean bill of health on the car, to get it sold whilst in good condition - the DPFs can be trouble on those Audis, apparently. I told him to buy a Skoda as being suitable for Czech! He bought a little Favourite, for £400. Bomb-proof and £3500 less than he sold the Audi for. Happiness around. Apparently the Audi did ten MPG better than the older Skoda, but £3500 buys an awful lot of petrol and simple maintenance on such a basic little car.
I think that way, and so even if there is something better, I am not curious to find it!
Best wishes from George