Bass- at what frequency does it change from sounding like a string to just a low tone?

This may help:

https://www.szynalski.com/tone-generator/

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.

Audio rusty: that makes a lot of sense to me. If you were me, where would you have your low pass cross over if your speakers dropped off six at 48?

@waytoomuchstuff IMO the way ATC rolls off the low frequencies is better than the usual way.  They roll off slowly starting at a higher frequency.  This is probably a marketing disadvantage but in most practical situations it's a plus.  A speaker that slowly rolls off the bass has far fewer troubling room bass problems while still being capable of great punch and greater dynamics than a speaker that tried to be flat to 30 or 40hz.  It allows you to place them where they're optimal for the rest of the frequency spectrum and fill in with subwoofers what isn't there.  I've got a pair of custom built 110s I bought used about 10 years ago.  I've had them in in at least 5 locations and they're very easy to just set and forget.  The 3 inch mid doesn't have the beaming issues a larger driver would so the in-room response is good and they don't upset room nodes nearly as much as you might expect.