Why even full range speakers need subwoofers!

Agreed, although I have no way to high pass the mains. I have blocked the ports on my Elipson 24F's and low passed the two subs at 45hz (12db). The mains are rated down to 30 hz (-3db) before extension and, in my treated room, sound better than without the subs. Still learning......

Yeah, he pretty much nailed it.  

Nearly 3 years ago, I added a separate amp and active low pass crossover to drive my woofers and the high level inputs of my active subwoofer, and installed an inline high pass filter between the preamp and tube amps that drive the midbass/tweeters.  That move not only improved the quality of bass, extension, dynamics, and control, but it also notably improved the overall clarity and soundstage of everything above that high pass filter.  The main amps are just loafing along from the vocal range and up, with no low bass duties at all.  One the best enabling enhancements I’ve made to my system largely due to the reasons he states in the video. 

The "don't need subs with full range speakers" crowd who say this either never tried it or didn't do it properly.  One thing that surprised me going down this path was how much timing between the mains and subs mattered—not just level and crossover.  Actually doing this may require reexamining long-held positions.

Once I got that dialed in, the improvement wasn’t just bass—it affected image focus and overall coherence in a pretty obvious way.  This seems consistent with @knotscott .  I'm running full range mains and 2 subs.  

It made me rethink how much of what we call “good integration” is really about timing vs just blending levels.

Curious if others have played with timing, or if most people are just setting crossover, gain, placement and leaving it at that.