Just for poops and giggles, I got a pair of Sennheiser pad adapters from Brainwavz. These little things are great. For ~10$ they allow you to experiment with any pad (that is big enough to fit) on your HD600 series headphones. I didn't expect this to work, I just did it for fun. And I was right, it was a fun experiment, but boy did it fail.
First pads I tried were the sheepskin HM5. Oh god. I didn't know the 58x could sound so bad. It sounded like putting a cylinder from finished toilet paper between the driver and my ear. Everything sounded wide, but like it was coming through a narrow tunnel. Not only that, but the FR was messed up. No subbass, very little midbass, peaky mids, shouty upper mids, hollow sound and harsh highs.
So I was like, it can't get worse than this. I was wrong. Next thing I tried were the HM5 pleather pads. They had the same kind of sound, but everything bad was turned up to 11. I'm not even gonna describe it. Take the sound you'd expect from a half broken piece of 20$ garbage, make it a bit worse and that's what you get. Brutal.
I also got a pair of microsuede pads. Now these, these are onto something. Let me be clear, they're still worse sounding than stock. But they're usable. Nasally mids, reduced bass throughout and somewhat harsh highs. They also make the headphones wider by quite a margin. But they're hella comfortable and look cool on the 58x, so I started experimenting. I put a piece of felt under the pad which calmed the highs a bit, so that was nice, the sound became a bit smoother. Then I thought I'd take off the back covers. It sounded somewhat more open, but nothing huge. So I went to the opposite direction and put a thicker piece of foam in the back. This killed a bit of the soundstage and made them sound semi open, but the result is not half bad. Again, worse by miles than the stock 58x, but it is usable and I could see me using it on occassion. Maybe movies would be nice with the added width. Or games. Or, you know, maybe my stock pads disintegrate at some point, I could use the 58x with the microsuede pads while waiting for replacement stock pads.
What is a huge benefit with all pads I tried was comfort. The 58x felt suddenly way nicer, more premium, way more comfortable. Then I hit play and forgot about it, but that's another story.
TL;DR : Don't do pads kids.