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

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.

My room has a slanted wall of windows in front of the speakers, closer to one than the other. Beside that is a rounded wall of windows. The view is wonderful--no chance of drapes. The ceiling is quite high, vaulted, octagonal, with long beams breaking up the surface. Three open areas are contiguous. There is only one wall to place a system and the speakers can’t be more than 18" from the wall to the back of them. The subs can only be next to the mains. There is no sidewall and only a partial wall behind one main. The other is next to a staircase. The wall behind the two mains is composed of different material. The other walls are veneered plywood.

We do have carpeting and the lower part of the windows are obscured at random with furniture, etc.

Despite all that, there’s really good off-axis sound, the speakers and subs disappear, and I’m generally happy with my system, which is a big step up from what I had before.

It’s an amazing home in a beautiful spot, no matter how anything sounds.

Piano = 27.5 Hz

Organ = 16 Hz