In my experience, increasing bore size decreases treble response. Also, I hate Spinfits with all of my life, it really screws with the tonality, plus getting a proper seal with them was always very difficult for me.
Symbio's are "generic" eartips with 3M 1100 Foam Ear Plugs (or some other brand) inside of them. It improves on Isolation, but tonality-wise they are like any other eartip.
Trial and Error, I suppose.
I think flc8s are designed for deep insertion especially with those long nozzles. Deep insertion has its own benefits over shallow insertion for example lesser but tighter bass and better treble response.
The stock tips that is one step bigger then the smallest size is perfect for my ear in terms of seal but not in terms of comfort. Spinfits cp100 s which is 11mm is smaller but gives perfect comfort. Spinfits remove some glare in the lower treble region that stock tips has and thus provide better details and pinpoint accuracy.
If jvc can beat them in this point then imo it's the best eartips for flc8s. Perhaps i should order the ms size because s with 10mm would be too small.