Velodyne's 'Deep Blue' 12 is a simple subwoofer that offers excellent performance, especially considering its low cost.
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B&W's DB4S is capable of superb deep bass response, but you may need to borrow a phone in order to calibrate it correctly.
With phenomenal sound quality and state-of-the-art room tuning circuitry, the Magico ASub is a subwoofer worth obsessing over.
If you have space, the budget and amiable neighbours, this talented subwoofer will slot into any suitable speaker set-up.