When you're out shopping for subs, be aware that while two subs may share the same specs (amplifier power, low frequency extension, etc) the chance of them sounding the same is pretty low.
Some subs I am familiar with, say those from Cerwin-Vega, have a "punchy" and "live" quality and sound great with rock & pop music.
Other subs, like the sealed designs from Martin-Logan, are on the other end of the spectrum and exhibit a rich & rather laid-back sound that still sounds good with rock/pop but IMO are generally more subdued than those from Cerwin and Velodyne.
And some subs have a little bit of both of the above qualities, Boston Acoustics for example.
Personally I like sealed a.k.a. acoustic-suspension subwoofers vs. subs that use a bass reflex enclosure because the former usually sounds cleaner and well, just richer, but unfortunately sealed subs are very difficult to find. Last time I looked ML sells some and most of Cambridge SoundWorks standalone subs used to use that design (CSW was started by Henry Kloss, who helped invent the first acoustic-suspension speaker, so that isn't a surprise!).
I think there are good-sounding subs out there from $200 to $400, but at this price point usually one has to pick from a sub that either a) goes low but doesn't have lots of volume or 2) has lots of volume capability but doesn't go as *low*. Extended bass AND high levels of it doesn't come cheap - that is not marketing but just plain ol' physics because moving lots of air requires gutsy mechanical & electrical components.
