My experience (and experience of others here), is that the slider for sub actually increases bass on the front door speakers (maybe also rear doors), and the bass slider increase the bass in the sub. Though not 100% sure, when I did the sliders, on sub, the front door speakrs 'vibrated' more on bass notes. I find the sub ok ish, but problem with A3 is, while the sub is decent when the car is sat parked, or in traffic, once it starts going over 25/30mph, or 40-50kmh, the road and wind noise drowns out the music in general, especially the bass.