My install of 8" Kicker Comp subs in my 2005 GTO.
I wanted to replace the stock 6.5" midbass drivers with something a little larger. I chose the Kicker Comp's because they were "Free Air" compatible. This was an important consideration, since I was not going to try and fit an enclosure under the speaker deck.
There are shallower subs, I would guess, that are also free air compatible, but not for the price of the Kicker's. I cut out the deck to accommodate the larger speakers. Using my Craftsman rotary tool and many cut-off wheels, it was a fairly easy task. I then needed to cut some spacer rings. To get the spacing I needed, I used two 5/8" rings. However, this made the system too tall for the stock deck cover to go back in place. I then turned one of the rings on its side and, using my jigsaw, cut it into a wedge shape. This worked perfectly! I then purchased some longer screws to utilize the original mounting screw holes, put lots of silicone under and in between the rings, and installed the subs.
I then wired up the 500 watt amp, installed the remote volume control, and found an unused spot in the fuse box under the steering wheel for the remote turn on lead. In the trunk, I decided to put the amp behind the carpeting, and on the driver's side of the space. It worked well, and is completely hidden. I will be monitoring the amp temps, however, as there is limited airflow.
I used Elemental Designs eDead v1 mat for vibration and sound deadening, then covered that with RAAM Audio's Ensolite closed cell foam for more sound deadening. I insulated the entire back and deck, plugging as many holes as I could. I wanted to get as close to a closed system in the trunk as possible. I then insulated the rear quarter panels. Finally, I moved to the front doors. I insulated the inside an outside of the doors. I used aluminum gutter screen to provide support over the large openings in the door, and then covered them with the insulating material. I also added some ensolite into the kick panels.
The sound deadening reduced my ambient sound level by 5 decibels when traveling 75mpg with the windows up. It dropped from 75 dB to 70 dB.
The subs are definitely better than the stock mid bass drivers. They are not, however, as tight as I would like, but this is probably to be expected from a free air installation. All in all, with the upgraded component speakers in the front, the system sounds much better than the stock system ever did. If I had to do it again, I would probably move the rear quarter speakers to the rear deck, and put the subs in small boxes in the rear quarter panels. This would give tighter bass, I believe.