A subwoofer takes over the low frequencies from the studio monitors letting them do what they do best, all the other frequencies. The choice to add a subwoofer to your studio monitors depends which kind of sound you are mixing. In case you are mixing sound for movies, TV or music with a lot of bass, you should take in consideration to add a subwoofer to your studio monitors. If you are mixing just your bands recordings, and you don’t have to accentuate the low frequencies, your studio monitors are enough.