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.  

@xcool  Seems to me that, because sub(s) complete the holistic frame of the original recording space, that which is not technically affected by frequency augmentation (say, 300 HZ signals when sub's top end is 120Hz) is instead placed in its proper sonic context. Often the most valuable qualities are the seemingly invisible ones we take for granted, such as foundation plantings in a landscape. Nobody pays attention to rows of holly, box, or yew -- all eyes are on the irises, the roses, the cherry blossoms. Yet remove the foundation plantings, and sudenly all the showy stuff looks naked, like a bunch of potted plants set out in a yard, no longer a landscape. So the landscape/soundscape analogy would be my best explanation.