What do you guys think of adding a subwoofer to this system?

I have tried subwoofers in the past but have always been disappointed.  I know subs have come a long way and would be interested in your thoughts especially people who have a similar system.

It's not really the sub as much as the room and the integration. The question isn't can you get a good sub, it's can yo make the room accepting of one.

Due to my  listening preferences I'm not a big subwoofer guy but they do add some heft and meat on the bones of your music. I don't have to have the lowest depths plumbed so I'm happy with a good eight inch that is both quick and articulate. 

I just ordered a SVS Micro 3000 sub. What sold me beside the good reviews is the ability to control the sub with a iPhone app. It also has an auto setup mode using a microphone. You have the ability to save different curves for different sets of speakers and because I'm old and screwed up, I don't have to kneel down or bend over to check settings. All controlled from my listening space. It's also not expensive at $899. 

It is best to use a crossover with low pass to the subwoofer and high pass to the mains. That means preamp/amp instead if integrated. Try just low pass to a subwoofer and complete signal to mains. If you cannot get a good integration then you know what is needed. Use two (or more) subwoofers, not one. I am using two JL Audio E112 (build-in crossover) with Quad ESL 63 mains, xover 80 Hz. In my room, 17’x35’ with high ceiling, I have bass flat to 28 Hz and no significant bass boost (at main listening position) in my setup with REW and a UMIK-2 microphone.

Which subwoofers disappointed you and where / how did you position them?

Have you considered B&W's own DB1 or DB2? 

Bob, unless you go to the extreme of buying a digital preamp with bass management and room control bypassing the low pass filter in the subwoofers, and get two very high quality subs you are always likely to be disappointed if you are critical with your bass. But, done right it would add greatly to your system increasing head room, lowering distortion and extending the bass down below 20 Hz. People will say the it is unnecessary to go below 20 Hz because you can not hear it. This is dead wrong. You can't hear it but you sure can feel it. Big venues breath at very low frequencies and with digital playback you can feel this in good live recordings. One problem with vinyl is that the very low bass is confused by record irregularities and to prevent your woofers from flying across the room you have to use a high pass filter below 20 Hz.