Which is a better subwoofer?

The construction and materials will be better on the Kef. It will also be a better sub for musical accuracy. The Polk gives you a lot for your money: Bigger cabinet, larger driver etc so will go a little bit lower in frequency response; the Kef goes down to 35Hz @ -3dB, the Polk to 28Hz at -3dB. Interestingly though despite the Polk having a more powerful amp and bigger driver & cabinet combination the Kef still manages to be louder. Polk is measured at 106dB max SPL compared to Kef's quoted 112dB. [Polk measurements here: http://www.hometheaterhifi.com/volume_12_2/polk-psw-505-subwoofer-4-2005.html ] Each 3dB is a doubling in sound pressure. The Kef won't sound 4x louder because of the way we perceive sound, but it still manages to do more despite a smaller driver, less powerful amp and smaller box. That's the benefit of better quality components and design in the Kef. Infact that's a major ace up the Kef's sleeve. It is a very very compact box. If you are limited with space then it's a good choice. Having said all the above, the Polk is still very good value and a decent sub for $250. It has the home ground advantage being an American brand which should make it slightly better value compared to European imports. If looks and size isn't a concern then for similar money to the Kef you can have the Elemental Designs A2-300. This brand is sold direct by the manufacturer so is exceptionally good value for the performance.

Of the two the Kef is going to be the better subwoofer, but if your going to spend that kind of money on a subwoofer I would look at something from the Velodyne line of subwoofers or HSU Research...HSU makes the best subwoofers in the world bar none and Velodyne is also considered a top contender especially when considering price into the equation....if you want a bargain sub that puts out a lot of sound take a look at the BIC F12 or the BIC PL 200.....more sound for your money than Polk....at least in a sub...