@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. 

SURPRISE,  SURPRISE !!  If anyone takes the time to read the product information offered by REL on line, they will understand that REL'S approach to sub design goes way beyond just adding lower octave bass.

-More enjoayble sound stage

This is something that does not get mentioned often enough. Although, it is probably more related to classical music recorded in a concert hall, more than other types of music, at other venues, or a studio.

There is a lot of ambient information carried in low frequencies, that add imporatant spacial cues. A bigger, deeper, better defined sounstage is the result.

I finally bought my first sub. It's a REL T5x and I love it.

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

My GF commented on how the addition of the subs really helped out the tonal balance in the high end- how the system became more relaxed.