Checking Your Setup
The first thing to check at this stage is that your cabling has been setup correctly. This basically means:
1. Disconnecting the red and black connectors for all speakers from the rear of your subwoofer and all speakers
2. Confirming the cables are correctly labelled and restarting, matching the red cable to the red connector and the black connector to the black cable at both ends, on the speaker and the subwoofer.
The cause of the subwoofer being out of phase is the wiring - your red speaker connections are connected to your black subwoofer (or external amplifier) connections and vice versa.
However you may still find the bass dampening still occurs – and that your cabling was all perfectly matched up.
In this case, you need to search out your the audio phase switch. This is usually found on the rear of the subwoofer or separate amplifier unit – these switches don't appear on all systems, but where they do appear they will alter the phase by 180 degrees, offsetting your subwoofer and increasing the bass.
If your system doesn’t have an audio phase switch and you find that your setup still has some bass cancellation, you’ll more than likely need to review your setup – some architectural or ornamental element could very well be causing the continued bass loss.