The only STAX that I thought bested the CRBN was the new SR-X9000, which does cost more and is arguably the best overall headphones I've heard to date if paired with a matching source.
For what it's worth, you can get replacement pads from Audeze from what I know, they just come with the glued pads pre-applied on one side similar to 3M double-sided tape.
I first heard the HD820 at CES in the Sennheiser booth where I still don't know why I thought it was so good. I ended up buying a set for myself because I couldn't believe there was a closed-back set that could look as nice, be built well, and sounds as great as it did. It does get the first two right but the tonality on my retail set was really bad and that's unfortunately the trend across the board:
https://crinacle.com/graphs/headphones/sennheiser-hd820/. EQ doesn't help much either since ultimately the drivers were just not as good with that particular assembly, and the HD800S thankfully showed otherwise. I sold them within a couple of months at a tangible loss. If you are unable to hear the difference between the HD820 (or do you mean HD800S that makes way more sense) compared to the HD 6XX, then I can't say much except that I am just glad you bought the better HD 6XX for a lower cost anyway.