JL make a great range of sub-woofers. Their base series is the WX range of 200Watt speakers start at under £60 (shown above).
They also have the W0 series (300 Watts), W3 Series (500Watts) and for real Bass lovers the W7 Series which are 1000W subwoofers.
For low-depth applications, there is the TW series, as shown in the photo below: –
They are available as component subwoofers, and also in a range of custom build JL enclosures as single and double subwoofers. The WX series are available in sealed enclosures
And the W0 series in ported enclosures.
And of course the ultimate W7 series in the perfect volume enclosure: