JL Audio 6W3v3-4 subwoofer 6.5" (165 mm).

JL Audio 6W3v3-4 subwoofer 6.5" (165 mm).

If you need a small-format subwoofer that really delivers in extremely small enclosures, this is where your search ends. 
The 6W3v3 is a true subwoofer, through-and-through. 
It employs the same technologies as its larger diameter brethren to deliver a whopping 0.35 in. (8.9 mm) one-way linear excursion. 

The 6W3v3-4 is best used with amplifier power in the 50W - 150W range. 
Optimized to operate in a tiny 0.15 cu. ft. (4.2 l) sealed enclosure, or a 0.25 cu. ft. (7.1 l) ported enclosure.