IMO there are at least 2 good reasons to use a sub(s) in a high quality 2 channel system. First, many otherwise excellent loudspeakers don't produce deep bass all that well. Second, for soundstage/imaging, the optimal placement of main loudspeakers often is not the same as the optimal placement for bass response.
Sure, trial and error is one way to get it right. For those of us who prefer to be a little more systematic, without spending too much time on set-up, I agree a test-tone collection might be helpful. Although, tones are pretty easy to find on the Internet.
https://www.audiocheck.net/audiotests_frequencychecklow.php
You can also generate tones/sweeps with REW.
Sure, trial and error is one way to get it right. For those of us who prefer to be a little more systematic, without spending too much time on set-up, I agree a test-tone collection might be helpful. Although, tones are pretty easy to find on the Internet.
https://www.audiocheck.net/audiotests_frequencychecklow.php
You can also generate tones/sweeps with REW.