With most studio-mixed music, in which the bass is often mixed into a single channel, one sub is plenty.
But if the lower frequencies are mic-recorded, AND mixed into stereo, two subs CAN (but won't always) make a difference. This seems to be particularly true with pipe organ, in which the phasing can cause some details to drop out when the bass is played back monophonic. But in these rare cases, even two subs can have the same result if not properly placed in relation to the room and listener.
Ultimately, YOUR ears are the judge. It's a good idea to demo a second unit, and spend some time at home experimenting. You might discover results contrary to what your most trusted sources preach to you, and your money ultimately should be spent according to your own experiential preference.
