My brother and I recently installed a 10" Kenwood marine subwoofer and 200 watt amp in the storage area of our 2006 Sportster SCIC. The end result is nice and well-hidden. The stereo has gained some badly needed soul that was lacking with two 6.5" components. Take a look.
Subwoofer
Amplifier
Subwoofer face
The small black storage bin still fits in it hole with about an inch to spare.