It is tricky, because everyone's ears are different and we hear different things. For me, I thought there was a–huge–difference between the spinfits and groovys. You should skim the thread a bit to get a bead on different tips choices and then experiment.
The three key factors, to my mind, are 1) seal, 2) placement, and 3)angle.
1) Seal: You want a good seal but, unlike normal IEMS, not completely closed off. I found a shallow insertion best, which, like you, meant using larger than usual tips. If you are getting "thunderous" bass but "recessed" highs, you might try pulling back slightly on insertion depth.
2) Placement: Again, more like circumaural phones rather than normal IEMS, placement matters. My i4s are nestled in the intertragic notch, outside, below, and forward of the ear canal.
3) Angle: My nozzles are angled up and back which send a clear sound into my earcanal. If I don't get the angle right the sound is lovely and warm, but clarity and definition are not good.
I tried an array of tips (+sizes) and manually adjusted the positioning of the nozzles until I got my desired sound. Then I worked backward to figure out how to stably fix the i4s in that position. I used a hair dryer and heat gun to carefully soften up the ear "hangers" and bend them.This was the only way for me to get the nozzles to fire at the right angle. I added foam from symbio mandarins into the groovy tips to get the balance between seal and articulate sound. I also lengthened the tips for greater maneuverability by adding a small length of rubber tubing which I sewed onto tips with kevlar thread. Finally, various silicone inserts on the ear "hangars" keep the whole mess in place.
I don't know, for me, this, along with DSP, brought the i4s competitive to some top tier phones.
Do you use LCDi4 with the concha rings? Would that improve the out of the head imaging more? thanks.
Paul