I think that you should use whatever crossovers sound best to you. If that is 40Hz or 60Hz, so be it. It is important, though, to distinguish between most music listening and 5.1 movies. Remember too, that your subwoofers will, in most cases, play frequencies below about 80Hz more strongly than most speakers will, and that the crossover isn't a brick wall. It is just a slope which attenuates the volume of a speaker by 12db per octave. If you set a 40Hz crossover for movies, and have a lot of low-bass content, your speakers may distort on some of the low-bass content.
You may or may not be consciously aware of the distortion, since the speakers are playing softer at 35Hz, or at 30Hz, than the subwoofers are. But, that potential distortion may still interfere with the overall clarity of the sound. In a calibrated system, which employs fairly linear subwoofers, and automated room correction, even subtle distortion may not be something you would wish for.
I think that there is some confusion about how bass works in 5.1 movies. The low-bass special effects in movies are not
restricted to the .1 LFE channel. The LFE channel was simply created as a way to add more
bass to the special effects in movies. Consequently, the Low-Frequency Effects channel is recorded 10db louder than the bass in the regular channels. But, the regular channels still play the same bass content that the LFE channel plays. That's an important point to understand.
What that means is that, if you are listening at -10MV, for instance, your regular channels will play whatever bass is encoded in the soundtrack at a peak volume of 95db, and your subwoofers will play the LFE content at peak volumes of 105db, and whatever bass is redirected to them, via crossovers, at peak volumes of 95db. (That obviously doesn't count whatever subwoofer boosts you are adding, or the action of DEQ, which boosts the bass in all of the channels and not just the subwoofers.)
So, using the example above, of a listening level of -10MV, you may really
not want your speakers trying to play 50Hz at 95db (not counting DEQ) much less 40Hz at that same volume level. I think it is very important to distinguish between the use of Large, or Small with a 40/60Hz crossover, for relatively benign music, (non-bass enhanced music such as EDM) versus the use of low crossovers for 5.1 action movies.
Those are completely different things with completely different potential results in sound quality. There is a reason that Dolby/THX standards have consistently recommended 80Hz and higher crossovers for 5.1 movie watching. As with everything in audio, there can certainly be exceptions to best practice recommendations, and I always believe in the concept of YMMV.
But, I do think it is important to point out why it is rarely advisable to set crossovers lower than 80Hz, and almost never advisable to set them lower than 60Hz, for 5.1 movie viewing at anything other than very low listening levels. And, even then, DEQ may not be your friend if your crossovers are too low, as it will boost the bass in your regular channels by quite a bit. And, that may cause the speakers to distort with some bass content. Good bass management for 5.1 movies is just a good idea.