As a general rule of thumb for listening to music, as opposed to watching films or playing games, you should get the best pair of active monitors you can afford which extend flat down to 50Hz or lower before looking at a sub.
As an example using top grade pro manufacturers as a guide. (ADAM, Focal, Genelec, Eve, perhaps Emotiva in the US) Ported 4" cones go down to ~60Hz, ported 5.5" cones go down to a very respectable 50Hz and the classic nearfield monitor with a 7" cone goes down to the low 40Hz (open E string on an electric bass is 42Hz btw).
So the claim from B&W that the 3" cones on their MM-1 go down as low as 38Hz is suspect to say the least. In fact it appears to defy the known laws of physics.
They aren't the only audiophile manufacturer to play cavalier with their spec and it doesn't make the MM-! bad speakers. It does make them expensive compared with equivalents from pro audio suppliers.
In fact a mate of mine who found himself pushed for space bought a pair of MM1 along with a small sub from a hi fi store and is very pleased with the result. He does look wistfully at my slightly larger ADAMs though and I think might make a different purchase decision today. Certainly most people who have heard both would rather have a single pair of A5X or even A3X rather than the MM1 and small sub. At least for music listening. If you watch a lot of films or play games it might be different. They would also be little more than half the price and twice as powerful.