Yes! I needed this topic. Finally a space where I can express my opinion 
Comfort can make the best sounding headphone useless. If it isn’t comfortable, what’s the point?
First of all, you mention the HD599… which seems relatively similar to the HD598. HD598’s aren’t anything spectacular in terms of comfort - they disappear on my head. Now, it depends what you mean by comfort. If you want a cozy headphone/ear-pads, go to Dekoni Audio, however, if you want a headphone that you want to forget is on your head, Sennheiser’s do a very good job at that. This has to do with several factors such as: clamp force, ear-pad density, ear-pad firmness, etc.
For me the clamp force on the HD598 is nowhere near tight, but if you have a larger head, you can expect more force - my head is relatively small, so I never had this problem.
For example, the HD598 is my main gaming headphone and it has been so for several years now. The headband is rather stiff and hard, but my hair acts as a cushion and doesn’t personally bother me. The pads are extremely soft, and I am confused how your HD599’s were hard 
My Dekoni Blue’s are extremely cozy. A headphone that never truly dissolves and disappears form your head - you can always feel them, but do not get discomfort at any point.
I have to say that iBasso (both SR1 and SR2) have the best headband, especially considering their price. The headband is very wide and is made of real Italian leather (you can learn all about it here, it’s Tuscan vegetable-tanned leather: https://www.pellealvegetale.it/en/vegetable-tanned-leather/).
There is something about the width and the softness of it, no need for cushion or anything. This type of headband is much similar to the wide headbands from Audeze, except this one is made of real leather and smells like real leather. I am not saying Audeze isn’t using real leather, I just didn’t have the time to look into their headbands… However, do consider that iBasso SR2 is just 500 bucks and has that.
However, another amazing headband is the one of Ollo S4X. It uses a self retracting suspension system. The way this works is by having two straps of either sides - these straps are attached to the either side of the headband holders (basically they are screwed in place, this way the headband is attached to the headband system). These straps are made of leather, however, within them there is a piece of elastic - and this is where the “real” headband is stitched to. It may sound complicated because I cannot quite explain it (if someone can explain it better, some help will do 
) - the headband is only stitched in one place, and that’s in the middle. Interestingly enough, this elastic is quite strong and it stays in place, I don’t need to adjust it. In fact, it is so strong that I can place it way below my ears (closer to my mouth) and it will not push back to its original place. This makes a very comfortable and accurate fit, it’s definitely something interesting… however, I don’t know how long the elastic will hold its strength overtime.
The best earpads are usually hybrids - thicker material like leather on the outside of the earpads, softer fabric material on the face of the earpads. Ears can get quite warm and sweaty if you are using leather/pleather on the facing side - they will also start sticking to your skin which makes things even worse.
I think this sums up my two cents. Conclusion:
Ear-pads:
If you want cozy ear-pads that won’t dissolve on your head - use ear-pads with memory foam, preferably hybrids (like Dekoni’s pads)
If you want ear-pads that won’t necessary isolate as much but will dissolve on your head - use soft velour pads (like Sennheiser’s)
If you want something in-between these two, you always have something like the Dekoni Elite Velour from Dekoni Audio - it is still velour, but it has memory foam “filling”.
Headbands:
The wider, the better
Cushining is fine too, it is more necessary if you have less hair because you will need some cushioning. If you have plenty of hair, then you have the unfair advantage of not giving a f*ck.
p.s. for a comfortable fit, it is crucial to both have the option of pivot (horizontal rotation) and tilt (vertical rotation). Everybody’s head is different, but most importantly, the ear-cups never sit at a 90˚ angle. Our head is “shallower” further back, so you will either need thicker cushioning further back, or you can just have the flexibility to rotate the ear-cups so they sit flush against the ears.
