One thing to keep in mind about subwoofers is the room resonance modes often dominate in the low bass regions. This is because the wavelength of a bass wave (20Hz = 56 feet) is about the same size as the typical listening room. So big standing waves can be excited in the room, which resonate and completely dominate an otherwise perfectly flat response in the sub.
Of course, since every room has different dimensions, every subwoofer has different a different sound simply by moving it into a different room.
The ways to handle this are to have bass traps, multiple subs (in the horizontal and vertical dimensions), or use a spectrum analyzer and graphic equalizer to electronically kill the bass peaks.
Having said all that, I personally use a single sub, without distortion-causing equalization, and am quite happy. Home theater is a little boomy, but most music bass notes are high enough that room resonances are not excited in my room. I use (and sell) sealed subs because they are much more musical compared to ported designs.
If you want my advice in musical choices for testing subwoofers... Bob Marley is great. On a good system, the bass is very tight, musical, and tuneful. If there's any problems with the subwoofer, it will sound boomy as all hell. The remastered versions of Catch A Fire or Exodus are primo.
