Keep in mind that the roll off on your main speakers and where your sub kicks in blend, typically at a 6 db per octave slope. I think it's a bit misleading to think that there's a point at which this transition happens. Yes, there's probably a point at which it's 50/50. A lot of the art to tuning your subs is to make sure that there's not a big hump at that point. You certainly want low base extension, but ideally it should be somewhat subtle, not just wow man the bass really is kicking like an MF right here. To that end, my suggestion is to listen to music that you are already quite familiar with that is well recorded. Whatever your go-to music is already that you would test any sound system with should not be forgotten. For example, there's certain steely Dan and Gustav Mahler recordings that I have been listening to for decades. Those are a great frame of reference for me because I can easily detect if they sound "better" than what I am used to hearing from them. A good well-tuned subwoofer should make its presence known without being in your face.
