I recently bought a pair of HD 559 earpads (part 572287) and a pair of HD 599 earpads (part 572273), and took the stock pads from a barely used pair of HD 560S (pre-2022 revision) for comparison. The 559 pads are identical to the 560S pads, and the 599 pads are the same in brown colour.

It's possible that the difference you're hearing is merely due to pad wear, but it's also possible that you have pads from two different batches/revisions, or a pair of genuine pads and a pair of knock-off pads. Are there any visual differences between the two pairs of pads?