Sorry, but I have to agree with @jawevofoy: Put simply, good sound means three things, 1. the IEMs deliver the whole spectrum from about 20 Hz to about 20k Hz, 2. they do so in a balanced way, 3. the noise floor is low, meaning you will be able to hear very quiet sounds along with louder ones. The louder you hear your ambience, the less you will hear of those same frequencies from whatever you’re listening to and the higher your noise floor will be, so you can only hear the louder sounds and miss the quieter ones.
And I don’t mean the quieter parts in a song (you could just turn the volume up for those) but quieter sounds in an otherwise loud piece (think the brushed snare drum in a jazz recording or whatever instrument is less loud in a mix). This is especially a problem for the very high and very low frequencies, that we can’t hear very well to begin with. That’s why so many people love “bassy” head-/earphones because they amplify those sounds that you wouldn’t hear otherwise in a noisy environment. It would be better to get IEMs with a better seal and just turn the volume down.
