Subwoofer for music, how low?

Still in experimental stage myself, but so far I am with oldhvymec, a decoupled, dual opposite firing box equivalent to two subs. Yeah, I too thought not much signal in Octave 0. Well, there's PLENTY down there; you control "boom" with SVS decoupling feet and lowering sub gain since a whisper is all you need. Main benefit is opening the mid on the towers. My own remaining problem is how to integrate two ambient subs in a difficult room. I'm trusting MC's advice that the key is not what, but where. Thx to OP for keeping this ball in play.

Walk around outside and look at a Real Time Analyzer graph on yer phone...everybody has one of those, right? Note the frequency range of the normal world...go ahead...I'll wait here. See that? In any case, subs (or the rare "full range" speakers) add the bit of "life" generally missing from playback without lower frequencies. 

Organ CDs with really deep bass

I use two REL S-812's in my stereo set up.  I have the crossover between 20/30 HZ and sit them right next to my Wilson Alexia speakers.  Did I NEED sub's? Absolutely not....however, they have added so much depth and soundstage to my system that I will always have subs no matter what type of speakers I own.  Goal is to have an array of 3 on each side (6 total) to further enhance the "live" experience.