I gotta agree, a servo sub will be the most musical. I spend a lot of time looking for a sub to go with my maggies. A servo sub was the only choice after listening to a whole bunch. If setup correctly you can hardly tell that it's there except for rock solid bottom. It was a hell of a lot cheaper than going for the big 20.1's to get that last 1/2 octave too!

You should be able to pick up either a Velodyne, or Paradigm servo 15" for less than a grand on AudiogoN.