Hi Sibiriak, here's my humble 2 cents: first of all, take the Sensaphonics seal test to make sure you have a perfect seal. Your seal may be lacking even if you think it's perfect, and that can exacerbate sibilance.
If you have successfully verified that your seal is correct and you're still experiencing sibilance, then try the following: cut two small pieces out of an eyeglass cleaning cloth (or any other thin non-woven fabric) and put them between the IEM's nozzles and tips. The cloth will add acoustic resistance and dampen the highs and sibilance, which will result in an overall darker sound signature for your 1plus2.
I've got customs for my Westone4 so i thinking to make an other for 1plus2. As I understand from the doctor which made it it's very easy to make preffered sound (less highs) via customs