Yes, 2 subs are idea. You also need a proper active crossover to place between your pre-amp outputs and the amplieifer inputs. Failure to use a proper active crossover, even if you match up the subwoofer low pass filter with the natural high pass frequency of the monitors, will result in much increased distortion at even moderate listening levels. The monitors to have LF relieved from them as the high excursion that would otherwise result causes strain on the mids since they are using the same driver.
A Behringer CX-2310 is a superbly designed/built unit. Despite it's cost, it uses only high quality parts throughout and offers transparent performance. It has infinitely variable frequency high-pass/low-pass xover frequency at a 4th order slope. With those small monitors, you want to cross around 100Hz. Dual subs are needed to do this as a seamless transition; one sub placed on each side of room, somewhat near the speakers.
A superior crossover is the Behringer DCX-2496, which has allowance of any crossover slope rate and type between 6dB/octave - 48dB/octave. It also has parametric EQ filters, shelving filters, dynamic EQ filters, delay settings, phase settings, etc. It will allow you to precisely control the bass SQ and apply some room correction filters too. Also, you can use shelving filters to carefully control the balance of frequency spectrum between the treble and midband/bass with a gradual threshold change and to do the same for the baffle step compensation amount - which the ideal amount varies on every different placement in a room and distance to listner; these are critical filters for adjusting most natural sound from any speaker system.
Buy high quality subwoofers. The SVS $400-$500 unit will offer much more performance(lower distortion and deeper
usable extension) as compared to the B&W. Get two of the SVS models to maximize value.
10":
http://www.svsound.com/products-sub-box-10nsd.cfm
12":
http://www.svsound.com/products-sub-box-pb12nsd.cfm
-Chris
