first all - 120 is way too much, I got my alpine type-r for 89.00 shipped to my door step.
Even with the box, unless something of quality is too much. A cheap box can be had for 15.00 and the type-s sub for around 65-70 shipped.
Any subwoofer requires an amplifier and also a good headunit to process a quality signal to the amp. If you keep the stock headunit, the signal will never be as good and something like this is NOTICEABLE!
Also, i thought the factory subwoofer was 8 inches?
Thanks for quoting me and giving me credit..otherwise i'd have to charge you royalty fees..lol