A few years ago, I went over to a friend's house to hear his relatively new MG-20's. After a few tunes I noticed a couple boxes behind and to the far right and far left of each Maggie. I asked, "are those subs"? "Why with those big panels would you need subs"?
My friend smiled and said, "you be the judge". So, we listened to a tune with the subs still on. Everything sounded wonderful as it had when we first started listening. Then he turned them off. To my complete surprise, the whole soundstage shrunk and the music lost much of it's weight and power and those little nuances and spatial cues that were so obvious before were now greatly diminished.
Those subs were REL Stadium 2's and I was so impressed with them that I ordered a pair of the newer Stadium 3's the very next day. They turned out to be one of the greatest upgrades ever.
I love the fact that you can take the signal right off your main amplifier's binding posts rather than having your main speakers go through the sub's crossover and they have all kinds of adjustment for gain, phase and frequency.
One last thing, whatever sub you decide on, don't let anybody tell you that bass below 100Hz is non-directional and you can get by with just one. If you're gonna do it, do it right and buy two!