If you use a BeoLab 2 with BeoLab 9s, you will end up with more bass, and deeper bass. A BeoLab 2 will raise most homes to their foundations if you really push it enough - it's massively powerful, great for music where bass really makes up a large part of the overall sound, such as club music and a lot of dub.
However, the BeoLab 2 does have it's drawbacks. 1, it is a separate subwoofer, and while it is usually maintained that bass below ~ 60ish Hz is practically undirectional (you can't hear where it's coming from), I find I almost always can. I'm not lucky enough to own a BeoLab 2 or B&W PV1, but I do have a Yamaha subwoofer. It's nothing special, but it does the job relatively well considering it's cost (around £160 I seem to remember). However, I recently switched from my large B&W DM7 loudspeakers (which would go deeper than a lot of small subwoofers) to my standmount B&W DM303s, which don't exactly cover the bass to the same extent (I believe they go down to around 80 Hz, and then begin to fall away). I tried using the Yamaha subwoofer with them, but I found even with the crossover frequency set to the lowest it would go (50 Hz), I could still very clearly hear where the sound was coming from (I can only position the subwoofer to the right of the speakers, outside of the 'stereo image' as a result of the way the room is laid out). With Classical and Jazz, it very obviously muddled up the sound, and just made the bass more 'vague' overall (not a result of the subwoofer lacking detail I can assure you). I found that it was slightly better with a lot of modern music such as rock & pop, but electronic and dub/reggae (of which I listen to a lot), it just seemed to distract me from what I was really listening to and didn't add much musically. Now maybe if I could place it somewhere between the two speakers it would sound much better, but whether or not I did this it definitely sounded better as I moved around the room (I often listen off-axis whilst sitting at my desk or whatever), the extra bass really added to electronic music. Great for movies too...
However, I still felt that the whole point in a sound system was so you could enjoy music, and the subwoofer didn't seem to add much...
When the 'subwoofer' becomes built in to speakers however, things change a lot. Part of the reason why I liked my B&W DM7s so much (despite their weaknesses) was the fact that each speaker itself could handle frequencies down to 30 Hz without any major issues, and I even found I could hear a solid 20 Hz tone. It was almost like having a subwoofer built into each speaker, and as a result, instruments such as double and electric basses, pianos, organs and even a lot of synthesizers (think Herbie Hancock's Chameleon and even some club music such as Tayo and Plump DJs) sounded absolutely fantastic down to the lowest registers.
Now as for the BeoLab 2 specifically, it also has it's own strengths and weaknesses when compared to other subwoofers. As I said earlier, it's very very very VERY powerful. It will go much louder than most other subwoofers I've heard, and also goes deeper than most subwoofers which don't take up a silly amount of space. A subwoofer of that size would realistically normally go down to about 35 Hz or so.
However, all this power and 'extension' comes at a price - it does tend to be a little 'slow' at times. Fast basslines and meaty kick drums usually sound quite 'wallowing' when a BeoLab 2 is used. A lot of people may actually prefer this sound, and it doesn't really make much of a difference when you're watching movies. In fact the extra volume is a huge advantage when watching movies, explosions etc... will make you jump from your seat.
It's all a matter of taste. Me personally? I'm fine without a subwoofer thankyou. I find even a small pair of bookshelves such as my B&Ws to be far more than adequate.
Here endeth the needlessly long response. I need to get on with some work! 