You may also want to peak at the subwoofer test results at HTshack.com
They measure for bass extension, clean output capability, and "tightness" (group delay). It's interesting and educational. SVS, Rythmik, and HSU all test very well, particularly for the price. Many better known brands fare much less impressively. In some cases, newer models may render some of the results less useful, so I'd look for models that either current or substantially similar to current models.
These test results may not be a perfect predictor of subwoofer performance in your system in your room, but in my (necessarily limited) experience, it's pretty reliable.
They measure for bass extension, clean output capability, and "tightness" (group delay). It's interesting and educational. SVS, Rythmik, and HSU all test very well, particularly for the price. Many better known brands fare much less impressively. In some cases, newer models may render some of the results less useful, so I'd look for models that either current or substantially similar to current models.
These test results may not be a perfect predictor of subwoofer performance in your system in your room, but in my (necessarily limited) experience, it's pretty reliable.