A single 8" sub in a box in the trunk will be a bit hard pressed to produce "noticeably better bass."
I'd also recommend using a 10" sub. The box will be marginally larger (some 10s will even work in the same .4ft^3 required by the 8W3), make better use of the amp's power (usually more efficient), have lower bass extension, and easily produce the bass levels you're describing.
If you're absolutely set on using JL Audio equipment, I'd suggest getting a 10W6. It'll operate in a .4ft^3 sealed box and can take 300w of power. Just make sure your amp is stable with a 3ohm mono load (the dual 6ohm coils on the 10W6 wired in parallel).
If you're open to other drivers (unknowns and underground), there are pleanty of options that will outperform a comparative JL sub for less money. Just post your budget for some suggestions.
