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

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.

I would set the volume on the sub with test tones and a DB meter for starters then adjust to taste after that. 

I set my subs ruler flat and let the wife listen. She said there was no bass?!?  So I adjusters it for her by taste and she ended up 10db hot lol!  
 

As for sub setup. If you can not highpass the -6 or -10db point of your speakers is probably the place to start. The -3 might have too much over lap. A test tone/DB meter will save a lot of messing around. 
 

I think a highpass is well worth the effort. You can then just pick a frequent where both the subs and speakers work well. I have found it is best to use the 3db point of the woofer NOT the speaker as a whole. For ported speakers this cuts out the port and for sealed it minimizes boundary gain and you can get a cleaner sound. All in all it does not matter as long as you enjoy it. I stopped carrying about “correct” a long time ago. You can have all the other audiophile stuff and a frequency response that you enjoy. 

Yeah, almost no normal person is going to choose a ruler flat response. It’s really only an hypothetical ideal. We all have preferences which are also affected by offensive acoustical issues.