The part of the audio spectrum that makes a note sound like an instrument rather than just a sound is contained mostly in the harmonics. Depending on your crossover point a vast majority of the harmonics are going to be reproduced by your mains and not your subs so it stands to reason that the lower the note the less definition there will be. Also if the bass you are listening to is being created by an electric bass guitar and that player is using flat wound strings you are going to get less definition because flat wound strings have a much weaker harmonic structure.
Bass- at what frequency does it change from sounding like a string to just a low tone?
I have two subs and speakers I like. I have little experience comparing them to anything else. As the frequency of a given note goes lower, at some point in my system it stops sounding like an instrument making the sound and instead it just sounds like the sound. I’m not crazy about this, but maybe that’s how it is for everyone?