I have had a pair of HD540 Ref (300 ohm) phones for about 20 years. I am surprised you find them to be uncomfortable - I sometimes have found them a little loose as they slide around a little but not uncomfortable. I absolutely love the phones and was always worried that if I bought new ones I wouldn't like them as much. Some years back I was able to try out a pair of HD600 phones. While it was difficult to get a complete assessment in the store, I felt that they were very close to the HD540s in sound. In fact, I decided that I couldn't justify the $600 price tag to get something that sounded so similar. I elected, instead, to repair my HD540 cable and ear pads. Well my headband has now fallen apart and can not find one anywhere so I decided to buy a pair of HD598s. These are quite different than the HD540s. But you can eq the treble up to get close to the sound of the HD540s for mids and highs. At the same time the HD598s give you a much richer, yet punchy bass than the HD540s especially at lower volumes. They have an excellent soundstage and are an excellent all around headphone. I use mine mainly for music and home theater. I think they would be a great gaming phone as well. They are comfy and light. They are slightly more closed than the HD540s. I haven't tried on the HD515s but the headband looks similar in shape to the HD598.
I have only had my HD598s for about a month so they probably are not even broken in yet. Still, I have come to like them more than my HD540s for everything except listening to a Lyric Soprano like Celine Dion. There are many threads here and on other sites that compare the HD598 and HD600 - as well as the HD650. From everything I have read, the HD650s would not be a good choice if you want to get close to your HD540s. Note the HD598s can be had for around $200 from Amazon (New) so it's not a bad way to go if you are not sure. I paid more for my HD540s back in the day!
Good luck.
