Compact subwoofer ideas?

I'm looking for a subwoofer but I'm not that keen on giving up boot space - suitcases and shopping take up space a lot of the time.

Has as anyone got any experience with an under-seat sub?
 
I was toying with this but then I heard the standard arkamys stereo in my new car. Didn't bother in the end. System was pretty damn good

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I had/have a Kenwood one. Hardest part is drilling the grommet in the bulkhead to fit the power cable through.
 

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