Well, I can't take any more pictures of my RSD at the moment, I'm a little indisposed at the moment, but I can answer a few questions.
The headrest part of the backseat, IIRC I didn't even have to take it out. We used the mounts for the frame that used to hold the back seat into place. When I get home (2 months from now) I'll take a look and confirm that.
As for the subs, I have a custom box built into the back of the RSD that comes down underneath the Mach 460 amp rack. It leaves you less room in your trunk, but compared to what I had in there before (just a box sitting in there), it's not so bad. I can now fit two sandwiches in the trunk instead of one.
The RSD is really quite easy to build yourself if you just take your time and do the measurements properly. Once you have main template traced out, you can be creative and use your own idea's to create new storage areas, mounting points for nitrous, etc. Remember, Piano hinges are your friend.
On that note, JT, what did you end up using to make your storage area more finished looking? I still haven't touched it, and I wouldn't mind some ideas for when I'm getting back into it.