I use both double filters and final e tips and that combination corrected any FR issues for me (I'm not treble happy in general) and actually made me really love this set. The new thing I tried with my olina recently is to EQ them to the HBB target using equaliser APO (using auto EQ settings I got from squiq.link) and that is something that even though I don't view it as a prerequisite I suggest you try as well at some point.