This bass booster circuit can be used filter the input signal as part of internal circuit of an active subwoofer circuit. This circuit is not only boost the bass (low) frequency, but also attenuate high frequency as well. You might consider this circuit as low pass filter, since the high frequency response is not flat, not really as expected in a bass-boost that only boost the bass frequency without cutting the high frequency.
You can see int he schematic diagram, that the actual implementation of this boosting is by attenuating the high frequency, then amplifying the signal to give the boost. This high frequency attenuation is done by the 100nF C2, then amplified by transistor Q1, and attenuated again by 100nF C3. [Circuit’s schematic diagram source: extremecircuits.net]