Oh, they mentioned it in their own thread:
https://www.head-fi.org/threads/review-german-maestro-gmp-400.702808/page-3#post-14125367
I did feel that the sound with the stock earpads wasn't quite as open and spacious as it could be. Separation could also be improved. It's very obvious when comparing directly with the DT880 velour earpads, although these are not a good match for the GMP 400 tonal balance... way to bright and thin sounding. Earpads that improve on these things while preserving similar tonal balance/linearity of the stock earpads are something I've been looking for for a long time.
I've been getting my hands on some no-name cheapo earpads for testing and came across a few interesting specimens for the GMP 400. No longer have the stock earpads for comparison, unfortunately, but I somewhat remember how they sound (bear in mind that I typically wear the GMP 400 without its cup enclosures, completely open config):
These leather earpads contribute to a warmer sound on my open/naked GMP 400. Despite the more laid back lower treble and more bass dominant sound signature (far from being bassy, though, still relatively neutral/well-balance) I find that the bass is punchier and just as tight if not more detailed with these earpads than with the stock ones. There's also more space for the ears than in the stock earpads (roughly same depth, larger width). Separation and spatial perception is either about the same or slightly better. These sound very nice, fit perfectly, are very comfortable and have great build quality. The best for those who prefer HD6xx-like tonal balance.
GMP 250 stock earpads. Not a fan of the material of these earpads. Seems to be of lower quality and prone to decaying quicker with sweat. However, these provide the most linear and coherent sound I've ever heard from the GMP 400 without sounding too bright. In terms of tonal balance, these sound slightly brighter than the stock earpads or about the same. Separation and spatial perception is improved despite the shorter distance between the ears and the drivers. These earpads are shallower than the stock ones but the holes for the ears are much wider. They sound the most coherent to me because the linear FR makes every sound appear to come from where it should. Consistent presence over the whole spectrum. The sense of depth/width might not be as large as the stock ones, but these cues are portrayed in a more accurate and effortless way. The sound breathes better, more naturally. These are the ones that might work best for my planned mods. They're very transparent without screwing up the FR. For unmodified GMP 400s, the stock earpads might still provide a more satisfying tonal balance.
Compared to the GMP 250 or DT880 earpads, the stock earpads sound like the music is being spatially compressed to boost some aspect of the midrange. This effect doesn't seem to apply to the whole FR equally, so they somehow manage to sound distant and upfront at the same time - a very odd perception. They're far from transparent, but the end result is quite pleasing and smooth as long as one doesn't dissect too much.
These velour earpads provide the most spacious sound. They sound airier with better separation and spatial perception than the stock ones. Once again, the ear hole is wider. Overall sound is laidback and also very coherent, but unfortunately, slightly too bright (not nearly as bad as the DT800 velour earpads). The treble on the velours, although somewhat picky/elevated, sounds the most natural to my ears (the difference to the GMP 250 synthetic leather earpads is not too significant). Major downside is the plastic strip on that wraps around the enclosure to attach the earpads. It's not at all elastic and is prone to ripping. Not great build quality either.
Again, most of these findings have been made with a "naked" GMP 400.
Dekoni's are tempting. I like their configuration, but worry that the treble might become pickier or too upfront, or that the bass-mid-treble transitions might not be as smooth as I'd like.