Finally Learned: Subs serve much larger role than adding more low bass

The title of this thread encapsulates my own experience. Several years' experience showed that my "subwoofer optional" Legacy Signature SEs simply couldn't play the Octave 0 notes present on some recordings. The gorgeous, enormous, discontinued Tannoy Definition subs being closed out at Upscale for 1/3 price, I couldn't resist. (This is after corresponding with Duke and realizing that the Swarm setup simply couldn't work in my non-dedicated listening room, and noting that RELs, like Model Ts, come only in black). 
So: the Tannoy crossover can be set at 31Hz, just where Legacys roll off steeply, then slope down at -24Hz/oct. from 31-120Hz. Result is a (nearly) seamless integration (current sub placement is temporary), with the sub not only playing flat & resolving bottom details down to 19 Hz, but also subtly enriching the overall sonic balance. So, yes indeed -- much more benefit than just fully resolving the Octave 0 content.  

These days, on the odd occasion that I have someone over or when I am discussing my setup with someone, I will mention that I have five subs. Invariably, they will make a comment about how my home theatre surround sound must sound awesome. I then tell them that actually only one sub is for the home theatre and the other four are for listening to music.  They are dumbfounded until I explain about how things actually work.    

If you like one sub, you will love four.

Getting 1 or 2 subs that integrate well with your system after having a boomy sub meant for home theater gives you an appreciation for what subs can actually do for 2 channel stereo.  In a small room you can get away with one good sub.

I'm an outlier apparently.

I had two JL Audio subs with their fantastic CR-1 crossover and also added DSP.

Nothing life changing.  Didn't do much for me.   Didn't find the added low bass to be significant and I preferred the tonality of my speakers without the subs.   Sold it all.  Happier with fewer ugly boxes in the room, fewer cables, less hassle etc.

(Great products...I just didn't need them to enjoy music).

I too am a subwoofer convert. I bought a REL S/812 last year and couldn’t be happier. One thing I also notice as it was mentioned many times in this thread, is that the sub also enhances the SQ of mid & high frequencies. I never understood how this works. I always thought if you set your crossover at let’s say 50hz, the sub only outputs sound at frequencies below the crossover.

Can someone help explain in layman’s terms why a sub can enhance the SQ of frequencies way above the crossover point?

Thanks very much.