Other members have posted observations similar to mine, but this bears repeating: absolute weight is definitely an important comfort factor--but so are distribution of the weight & efficacy of the headband/leather-underband design.
My 2 ZMF dynamics (blackwood Aeolus; silkwood VO) have a very particular combination of relatively high weight (definitely high weight for the blackwood Aeolus) + superior comfort in the headband design. And I'm talking stock headband--no suede underband or Pilot pad. Those 2 products are definite potential comfort upgrades, which is nice to have--but thankfully, the comfort I feel with the stock headband is really superior.
I've had lighter headphones that were bizarrely uncomfortable compared to these ZMFs...weight is a factor, but not the only factor.
I hear what you're saying. But here is the question - are we talking about two different things?
One is comfort during the first minute, first hour, first three hours, meaning when I put a 600g ZMF on I might feel just right, perfect weight distribution, proper fit, right clamp pressure. When I put on my HD-600, even after 3 years of use, the clamp is stronger and so the comfort is not as good in that respect.
The other is lower back of the head and/or neck pain after prolonged use, after four hours or six. I may not think the Sennheiser has the best comfort with that clamp, but after six hours I am going strong with no pain and have even gotten totally comfortable with the clamp. But after four or six hours the 600g headphone is starting to affect the lower back of the head / neck.
My long-term (owned) experience with heavy models was a 550g Audeze LCD-MX4 which was fine after a few hours even, but not after six. And a 620 or 640g LCD-2CB which was murder after one hour (and sounded bad too in retrospect). How does a 600g ZMF feel after four hours?
Of course there is an argument that one doesn't need to use a headphone that long.