The subwoofer should exclusively handle the low end. With the Y-cable in the SONOS CONNECT's Line-Out connected to both amplifier and subwoofer there is no crossover for the main speakers and they'll push out bass too. This is an unwanted overlap. Some subwoofers have a built-in filter (crossover) that will provide this function for their line and/or speaker level pass through. In a pinch you could add an inexpensive car audio crossover to split the Line-Out into Low and High frequency bands.