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

There's not a lot going on below 40hz,except maybe electronically enhanced tones or pipe organ tones.What there is normally are low tones and resonances. If your floors and walls are "singing" along than those tones won't sound as clear as you'd like. This is a link to some frequency charts that illustrate where the instruments and vocals reside that might interest you.https://www.bing.com/images/search?view=detailV2&ccid=fjNn8kLi&id=981426F355CF8B0314AEA0066F324A52613C25B5&thid=OIP.fjNn8kLitMiphe7sH3ZolgHaIR&mediaurl=https%3A%2F%2Fexternal-preview.redd.it%2FG-ZpqOjDR1r1xMXzByqv2OX0cRK5KXIpz3VGMfXYpkw.jpg%3Fauto%3Dwebp%26s%3Dfe5718bae0049cc23247e7345c320538c4fe93fe&cdnurl=https%3A%2F%2Fth.bing.com%2Fth%2Fid%2FR.7e3367f242e2b4c8a985eeec1f766896%3Frik%3DtSU8YVJKMm8GoA%26pid%3DImgRaw%26r%3D0&exph=2151&expw=1924&q=frequency+chart+for+instruments&simid=608000213540172012&form=IRPRST&ck=98CFCCC39A2D2047CC2604D79D0D6323&selectedindex=7&ajaxhist=0&ajaxserp=0&vt=4&sim=11

I like the sound of sine waves from 20 Hz and up. Billie Eilish is very good for this :-)

Bed aware not to set the level on your subs too high or you will only hear the "sound". And use dsp.

I have ATC SCM40 mains, 2 SVB3000s, along with an A308 MF amp, a good streamer, and a good DAC. I also have a very challenging room with no options to improve anything at this point, room-wise. I don't know if I've ever looked at my profile.

So I gather that for the most part, the lowest sounds aren't within the range of  instruments per se, and the clarity of these is room dependent.

Thanks for the links.

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.