Were you listening to the subwoofers, or listening to a speaker setup properly blended with the subwoofer?
The subwoofer itself, if set up properly, won't sound very good *on its own*. It isn't meant to. It's only meant to reproduce low frequencies. All the higher harmonics, etc. shouldn't be coming from the sub, but rather the main speakers. So, a sub, on its own, won't sound too "snappy".
The Monster subs are listed as producing rather high frequencies, which, if properly calibrated, they won't be producing in an actual system setup.