Never had 600s but IMO this is a subjective thing. Had absolutely cheap amazon pads that positively affected my 6XXs. Note had to mod to fit of course. Definitely should watch videos about pad swapping on those specific cans and how different materials lean when it comes to FR. Find something that leans the way you personally want the sound to go. As u/Hdjekso says ZMF makes Fantastic pads and while I haven't used them on the sennis I do have the perforated lambskin universe on my Open Alphas. They are great pads for those cans. Good luck in your hunt. To me the pad swap journey is the fun part. Though you can drop a lot of dough on pads if you aren't careful.