I actually bought the FXC80 just based on my positive experiences with the FX67. But unfortunately, I was not a fan of the overly bright sound that DID NOT respond to EQ at all. The FX67 just has a better presentation, and nothing fatigues you in the sound. Treble is no problem at all. Plus, it responds very well to EQ'ing. Try boosting the mids if you find it not engaging enough.
Another reason I absolutely love these. I see that reaction often due to their price, "Why not?"
Try the stock tips first just to see if you like them, but Hybrids simply inserted better and had increased isolation. Between them and the marshmallows, the Hybrids were more comfortable and obviously were much either to insert and take out. Foam is kind of a pain in the butt, but at least it's not comply foam! And as mentioned, I find the marshmallow tips superior to comply foam.