If I buy an expensive pair of full range speakers that has audible bass distortion at the volume levels I want to listen at, that's a problem. The solution to the problem shouldn't be adding a sub and crossover so that my expensive full range speakers can have the bass filtered out so I can treat them like I bought a pair of monitors. I want to hear the bass I paid for in my expensive full range speakers and not neuter their bass by filtering it out.
All speakers have a limit to their bass output for a given frequency. Can't violate the laws of physics. "Full-range" speakers may play low, but that doesn't mean they can play low at higher volumes, or without an increase in distortion. Continued playback of low frequencies at high volumes also heats up the voice coil resulting in
power compression / thermal compression and the accompanying distortion. So while your full-range speakers may play fine 20Hz - 20kHz at 80dB, that's unlikely to be true if you are asking it to play at 95dB, or even higher.
The first image in the
Audioholics Subwoofer Measurement Standard Part I article is a perfect example of that. The manufacturer would likely state the subwoofer measured in the image has a frequency response of 14Hz - 120Hz, and maybe also state a max SPL of 100dB. But at 100dB the frequency response is only "flat" to about 25Hz. Separate from SPL, low frequency distortion can bleed into the higher frequencies. The same issue applies to woofers in a main speaker and as you would expect at a higher frequency.
To address that, you could either go with speakers that have much greater linear bass displacement capability (i.e. buy a physically bigger speaker with correspondingly larger woofers that have better Thiele-Small parameters), or integrate a subwoofer. The former is a more expensive solution than the latter, but of course it is something you could go with. Besides being expensive, it does also limit your choices since there are very few speakers that a truly full range at louder listening volumes. For movies or bass-distorted music or pipe organs or the 1812 Overture, you will pretty much need a subwoofer, or a main speaker that is essentially incorporating powered subwoofers. Especially if you want to play back at, or close to,
reference levels.
For what it's worth, I have set the crossover point for our pair of Magico M2's at 50Hz when used with subwoofers.