Yes, this is a tough one! It can be a bit overwhelming at first.
My sense is that people tend to gravitate towards a handful of tips that typically work well for them with certain kinds of IEMs (e.g., I mentioned above using foam tips for deep insertion IEMs and silicone for shallow). As a result, some folks have a go-to selection of tips at the ready for trying out new IEMs, and they tend to know how those tips will likely interact with IEMs.
I’ve found, for example, that Azla Sedna tips work well for me with shallow-fitting IEMs like the Campfire Solaris 2020 and the Andromeda 2020. For the most part, the tips have predictable effects on the sound; the regular ones make both these IEMs sound slightly thicker, richer, and darker, with more bass emphasis; the light ones, by contrast, make the sound a bit less bassy and brighter. (Others have reported the same thing; so it’s not entirely about individual preferences - or at least it isn’t always).
In other words, once you’re familiar with the tips, you can dial in your choice with an IEM to suit your preferences. There can certainly be occasions, though, when your go-to tips don’t produce the desire result; then you may have to experiment further.
The trick is not really having a baseline for knowing what the IEM should sound like in the first place. If you look through threads on various forums for the original Campfire Solaris, you’ll see diverging views about the sound based on whether people were using foam or silicone tips. Sometimes, a consensus about tips can emerge.
It’s good to read around, then, to find out which tips others are using and what they’re hearing with them - for this reason, it’s best when people giving their impressions specify which tips they’re using! Otherwise, as you note, impressions can be all over the place!
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Re. finding the right Comply tips: if the Comply charts don’t list your IEM, you can check on head-fi - do the search within the thread option to save yourself from scrolling through gazillions of posts! You might also email Comply!