So, driver 'coherency' could refer to many different things, so this runs afoul of
the private language problem - it's good reading on this topic actually. But when it comes to 'bass quality', Listener and I have both given explanations for how
frequency response at the ear drum is ultimately responsible for goodness/badness here. We're NOT saying "measurements on a graph shown relative to a target
exhaustively explain subjective goodness/badness for bass quality", even though you definitely can glean some of that - or predict some of that from the graph. But let's go through those points again here:
1. FR plots shown on 711-based systems don't provide an accurate picture of how the IEM performs on a human due to the less than accurate acoustic impedance, yielding inaccuracies at low and high frequencies. Simply put,
you cannot expect to get a good read of the FR in the bass and upper treble for IEMs with most measurements graphs found in the wild. But that doesn't mean it's NOT FR that's responsible for your experience.
2. Tones that are dominantly experienced as
in the bass still token a wide range of frequencies outside of the bass range. So when you hear a kick drum, for example, that tokens way more than just the bass frequencies, but you may associate its goodness more commonly with "bass quality".
3. Understanding this involves a more careful analysis of the FR for the relationship between various frequency ranges that impart goodness and badness to the tones you're experiencing. And quite frankly, this is not something I'd ever expect someone to do - but it is possible. A good example of this is the "soundstage effect". This is uniquely to do with a relationship between mids and lower treble. Pull back on the mids and increase the lower treble, and you get this widening effect. Do the opposite, and things sound closed in.
By all means, believe what you want - but I've also been there. What helped me understand this stuff was doing tone generated sweeps and EQ. Even if you don't agree with me on this, take this consideration in good faith - try some of this out and see how things change for you. Yes... we have the luxury of state of the art measurement rigs, but manual tone-generated sweeps will reveal a LOT about how headphones perform
on your head, and the differences with how they graph. Try getting into EQ to see how changing some of these key regions affect your perception of some of these intangibles.