The HD600/650 is the only headphone I bought and sold 4 times in total (combined between the two; 650 3 times, 600 once). Each time was due to its comfort which I ultimately couldn't stand no matter what I tried to make it more comfy. I always loved the sound of the HD650 in particular (more so than HD600 as it didn't have that slight 4k peak that bothered me a bit sonically) and if it was comfortable enough for me it honestly would have never left the stable because I really love the sound of it that much. But each time it caused so much pressure on my jawline; it laid against the points in my jaw that ultimately caused them to ache after about half an hour mark, and cause a headache by the hour mark. And no amount of stretching of the metal band could fix the problem... I literally once bent it so much that the bars actually had a visible angle in which they bent outwards... this helped a bit, but over time the pain from the pressure just came back. One problem I noticed with it was that it doesn't really have much much of a horizontal swivel, so I couldn't ever relieve the pressure far away enough from my jawline. But ultimately I realized that the main culprit was the oval shape of the cups; practically any headphone with similar oval shape caused the same problem for me and that includes the lower Senn models like 558 and 598 which clamp less. They just lined up badly with the part of my jaw that was the most sensitive. Funny thing is that the LCD-2's, which has as much clamping force if not more than HD650, is heavier, and in general is often said to be MORE uncomfortable than the HD650, was perfectly comfortable for me... and that's when I realized that it was shape of the cups that didn't play well with my jawline and not really anything else. It's really a shame because I honestly really do think that these models sound top notch.
So my recommendation is to try different length of adjustments, play around with positioning of the cups (try wearing the headband further back of the head or more forward), and also to bend the metal bands outwards to relieve clamping pressure and see if it gets better for you. If it doesn't, then you may just have the same problem as I do... in which case I don't think there is much you can do about it, unfortunately.