Subwoofer Recommendations

@maxwave, to an extent that is how it works ... the area of a driver (circular) increases exponentially with radius but a greater linear displacement also helps to generate bass at low frequencies.
The cone shape is also a factor.
One > two drivers double cone area and add to symmetry (power output).
Two > balances internal energy.
If you get hung up on 'driver envy' consider the fact that @ 40 Hz the wavelength is 8.6 M or about the same dimension of an interior wall (large room).
@stereo5, sealed enclosures have relatively good output response and if 
given good power will perform very well.
KEF produces extreme HiFi Gear and I personally would not hesitate to trust in the engineering some.
A single sub is smart ... not a mistake.
+1 @nonoise, good review article btw.
+1 @ozzy, Hsu is also very good and possibly a more affordable model.

How did you decide between the sealed vs ported sub on the SB2000?

The main reason was you were only looking for one sub so thought space may be a consideration and two SBs are a lot smaller than two PBs while still getting you under 20Hz (-3dB).  The other is I tend to prefer using a sealed sub for music, but I understand properly-designed ported subs can work as well.  Last, I own an SB2000 I use for HT and it alone can shake my living room when required so two PB2000s seems almost like overkill for music purposes.  So that’s where I was coming from with my recommendation. 

@rego
Thanks for the explanation

I have two SVS SB 2000’s, as I listen mostly to music.  The ported SP 2000’s provide more boom and were noted to be better in a home theater for movies. 

@stereo5 , you will a 12" sub to produce anything useful below 50 Hz. Just a caution, bass travels...far. When my system is on playing something with a lot of information below 30 Hz things on the opposite side of the house rattle and buzz like crazy. This may cause trouble in an office environment.