I got some stuff from Moondrop today, and since I think someone (@IEMusic
?) asked for impressions of their filter on other IEMS I've also ordered the KXXS' filters to try it out on FDX1.
The first thing to notice is the filter won't fill completely the FDX1's nozzle mesh, I'd say it covers around ~90%. The grill screams quality compared to the similar plain metal mesh grills that I got from Aliexpress. The damper material seems cotton-ish instead of nylon-ish and they added an extra anti-clogging layer, so very likely you won't face the moisture build-up going quieter issue using this filter.
I've tried to capture their dampening proprieties against the light, as you can see it tames the light quite well.
(disclaimer: BOYA mic + vinyl tube. It's not comparable to IEC711 measurements but for modding proposes only it does its job in capturing the changes quite well)
I use the green damper on FDX1 regularly (in my case the one with the most dampening material), so I used that for reference. I thought the filter would only tame the upper-mids just like the classic micropore tape but it also takes a bit of the mids (around 2dB) too and it keeps the treble res a bit better than the green.
The first impression is very positive. I like the green filter the most because I think the mid-range tonality in the stock white filter config goes shrill very easily at loud-ish volumes, despite it having more details I prefer to sacrifice the details a bit to get a more correct mid-range presentation with the green filter. With the Moondrop KXXS' filter and the 3-4 songs I've tested so far I perceive the same increase in warmth but more details compared to the green.
So far so good, If I get new nuances I will update this post.