Thank you for the suggestions, i'm already using a copper cable from Null Audio (Arete MKII) and it appears to smoothen out treble harshness just a tiny bit.
I don't think I am willing to dish out $1.3k for an uber cable, would rather get a nice pair of CIEMs with that money :)
I've read up a bit on the Ref1 and they do seem pretty enticing. I may consider them although that would depend on selling my 1plus2 first.
What's the main source you're using with your 1Plus2? I've found sometimes the source can be the culprit in causing sibilance.
What happens if you run them from the FiiO X1 for example? Any less sibilance.