I used a piece of quite thick felt which I removed of modified a HD681-EVO.
Tried various felts and creatology felt types but that Superlux felt really did the trick.
The felt is 3mm thick and quite dense.
It even makes sense to buy a HD681-EVO for this and buy some Chinese pads to use on that one and you have 2 fine headphones.
As the dip around 5kHz is actually a resonance, that dip is much smaller as well and the highs are now silky and behave quite well.. bye bye sibilance !
Trouble was... the bass became relatively lifted because of the highs were too tame and it became too dark for me.
From 200Hz to 3kHz there is a 5dB drop which needed to become smaller.
This was my 'first attempt' and will try to tweak it further.
That is done electronically with a very simple filter that consists of a resistor and capacitor in series.
When I am completely done with it I will make a pdf about it.
Wouldn't surprise me if the bad sounding K550's could benefit from the same mod as well.