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

Gosta: I have a hell of a time determining where to set the sub volume. Which result reflects the recording's intent? How did they mean to balance the bass with the rest? And what sounds the best to me? you'd think I know that last one, but when it comes to bass... I wish I could discern all this better. Anyone else's experience deciding volume would be welcome.

You shouldn't hear the subs, at all.  They should just make your speakers sound enormous, in terms of dynamic range as well as depth.  It's hard to tell what you are describing but it sounds like really poor integration.

First, check the placement.  Look at the AM Acoustics room mode simulator and try to keep your subs and speakers out of the lowest octaves.  Next, measure.  Use Room EQ Wizard or OmniMic to see what the response looks like from your listening room.  There should be no gaps or excess peaks, but listeners often prefer an elevated shelf below 100 Hz.

String tone of any basic frequency is always a function of the fundamental and it's harmonics which vary from instrument to instrument. Otherwise a given frequency, bass or not would sound the same from every string instrument. To speake about string tone without the harmonics, or indeed any instrument without its harmonics makes no sense.

@m669326 

I have ATC SCM19's, Musical Fidelity M6si, Lumin D1 streamer and a Denafrips Ares II dac.

I use four subs connected to the MF via MiniDSP 2x4.

I am still tweaking the setup because the speakers a relatively new to me and I have reconfigured my listening space. Despite this the low  end sounds very good to me, but I certainly appreciate your frustration. There is such a range of bass content and quality in music that it's hard to tell if an imbalance is your system or the sound mix. Either way I don't think anything but the lowest tones should be unrecognizable or boomy. If they are, it might be a result of the placement of the subs, the room itself or a combination of both.

Also, I am curious about what you feel is a "very challenging room"?

The basic method mentioned by @erik_squires is going to be the most productive for most people. But you can get an idea of what your low end frequency response curve might look like by using something like the "Tone Pak" by Bass Mechanik just to get an idea of general areas where your bass is to loud or too soft. It’s basically just level normalized test tones from 2hz to 50hz. Tedious, but helpful and ultimately educative for most people, I would think.

Note: don't keep a single sine wave going at even moderately loud decibel levels for an extended period because it can damage your sub.