I did the 6X9 Sub Install
I did not want to do all the cutting and modifying that is required on the 8 or 10 sub. So I just bought the Part Express 6x9 sub
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=264-837
and put in a Alpine 4/3/2 amp under the drivers seat. I bridged the second channel for the sub and ran the crossover at 125 from the head unit. I run the other two channels for the front Boston Acoustic's. I did not want to shake my family jewels as I listened to my music. I just wanted to feel and hear my music with my windows rolled down while I drive down the highway.
Here is what that the parts express sub looks like to the Subaru sub side by side.
Much larger magnet to withstand the power from the Alpine amp. I am pushing about 50 watts to it. at about 3/4 power and 5+ on the head unit.
New Sub mounted.
I had to use self tapping screws because the originals were in a different location. The sub hole was a little small so I just cut the excess away and shoved in the new sub.
The initial sound difference was amazing. I am so happy that I went with this solution. It is enough power and it totally hidden. Keeps my trunk available for all my bike racing gear. I don't have to worry about damaging my bike gear or my bike gear damaging my speakers or amps. Plus it was only $40 shipped to my door.
And my iPod has all my music so I am never looking for Cd's..