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

I have found many better examples of a double bass rendering lows but these are seldom even played.
https://youtu.be/ksQ_jTrk_o8

That first one actually give a graphic of what you are hearing. This is similar to what I watch using the AP 'DecibelX' while listening.

https://youtu.be/NqR33Dy0a-w

Some Chinese' Drums
https://youtu.be/w2S-A3Cw3sw

If yo jump in here at abut 52 minute point you will catch a lot of 52 HZ and nearby harmonics and I even saw the graph dip down to about 45 Hz at a few points.

A Lot of Electronic Bass tossed in just for fun
https://youtu.be/de1yI54LZWQ

At the level of equipment I use, what matters most to me is if I can discern actual notes instead of just low rumbles and thumps. One of my go to sound tracks for bass is One od Emmylou Harris's more obscure albums called "Thirteen". She does her version of Bruce Springsteen's "My Father's House" which has a synthesizer doing a pretty convincing imitation of a pipe organ.

Speaking of thumpers, I'm glad to see that the Hip hop car thumper bass craze has mostly subsided.

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?