Alright lecture time.
The amount of mid-bass that you can have before it “bleeds” into the rest is actually also affected by the rest tuning. Example, take 2 identical iems except you have a high shelf difference of 5 db from 2k+ With a boosted bass shelf.
IEM 1
IEM 2
And you have 2 people, A being extremely sensitive to anything bright (aka mids/treble). And B being extremely INsensitive to anything bright.
When A listens to iem 1, the low amount of mids/treble will perfectly balance out with A´s hrtf (sensitiveness) and thus the amount of mid-bass will also not sound muddy.
However when B listens to iem 1, the low amount of mids/treble will be way too little for them and it will sound dark af to them, and now the bass quantity is gonna be a detriment as well and thus make it sound like it is bleeding into the none existent mids/treble to their perception.
But when A listens to iem 2, now the mids/treble will be too much for them and the bass is gonna be perceived as much lower than it was before.
When B listens to iem 2, now the bass will not be bleeding into the rest because the mids/treble is finally gonna match their HRTF and perceieved as perfectly balanced and not dark.
Is everything clear to you now?