All subwoofers will require an amplifier. So make sure you have the space/money for the amp too. The stealth box above is great so long as you get a slim line amp that can be mounted behind the boot side panel.
The line-outs from the back of the head unit are un-amplified, they just provide the signal, a bit like the output from your cd player or sky box at home.
Hope this isn't a granny sucking eggs comment, but thought it might help some people who aren't into audio stuff
The above mentioned Blaupukt box - I have some for £50 but you will need an amp - I've got plenty of those
p.s. - that box - you will have to bend the grille to get it to squeeze into your boot
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