@m669326 To answer your question, any frequency. When you listen just the bass, it most likely not going to sound natural. For example a double bass can go as low as 31hz(E1), or 5 string guitar. Almost every instrument has overtones that make is sound like it is intended to sound, natural and realistic. The same concept applies to sub and speaker integration. In a well integrated sub should give intention to self.
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?