(I was replying to an earlier post and probably wouldn't have bothered to comment but it might apply for someone else where the point isn't moot

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While my own experience would make me tend to agree that a search for the optimal tip (and not just including foam tips) could very well result in finding a better seal and transformed SE525s, as far as I understand it, the frequency response of the KZ is simply more bass heavy and has sub bass representation that the SE525 just does not. The SE525 is more balanced. The KZ is also tuned for "young ears", being defined as having bass and sub bass suited for current music (and entertainment/games) and high end that can't even be heard by 45+ year old ears.
I think that an understandable mistake with foam tips on different IEMs with different size nozzles is to assume that tips with the same outer diameter will result in the same fit and seal for both, when that's not necessarily the case.
What happens with the material on an IEM tip is that it conforms to the contours of the ear (direct quote from Captain Obvious). The same outer diameter on tips for large nozzles, like most chi-fi IEMs (also very obviously) means less foam material to do that conforming than small diameter nozzles.
The result is that they look the same but are not the same, and with the wide variety of ear canal sizes and specific shapes, this can make a meaningful difference. Ear canals tend to be tall, boxy-ish ovals, not round. IEM nozzles and most IEM tips are round. The diameter if the KZ bare nozzle itself is pretty close to the average ear canal width, so in most cases, adding any size tip's sleeve and tip material is going to mean that no matter what size tip is used, the fit will be "snug" from side to side with the foam filling in space on the top and bottom. For most of us who haven't made a highly detailed study, results are going to have some unpredictability and require some trial and error.
I also think that trying to contrast and compare IEMs as a group discussion subject is problematic. We don't all hear the same thing. We don't all define what we hear in the same way. So the back and forth about various aspects can be futile with us realizing it.
I have "tested" (
The Ultimate Headphones Test) KZ ZS10 Pros, listened to recorded music and tried them at rehearsals and a gig. For me and my ears and my preferences, they have more than sufficient bass, which I really only find apparent with tests and recorded music with sub bass, and by comparison with others I've tried have a harsh sounding upper end. I don't say high end because I have 50-something ears and know there's content I'm not hearing there. I don't find them particularly well-fitting/comfortable...they feel bulkier than they look to me. While I suppose I would find them useable if they were somehow the only choice, I find that the dealbreaker is they have the least isolation of any that I've tried so I had to run them notably higher to have the same balance, so given choices, I wouldn't use them for playing live.
An anecdotal case about tips that might be worth watching...