I guess you didn't need the extra power from a 1 ohm load since you're happy with the output already. When I say specs, I mean how much airspace is inside the box and what tuning frequency. CSCStang is a great guy and knows his stuff. I had him build a box for a friend of mine and he absolutely loves it.
Trunk rattle is inevitable when dealing with big bass in your trunk. Once you add mass to it aka sound deadening, it will harden up like a rock and make your bass sound so much cleaner and will increase a bit more SPL actually. The vibrations are pretty much getting soaked in the metal and creates a rattling noise, which is super annoying. Adding deadener to your trunk and car doors make a huge difference in sound quality so I suggest you do that when you get around to it. There's many brands to choose from. Just depends on your budget. You might wanna try Stinger roadkill for its bang for buck value.
Amazon.com: Stinger RKX36B Roadkill Expert Series Sound Damping Material Bulk Pack: Car Electronics There's also
RAAMaudio - Automotive sound deadening products or
Vehicle Sound Deadening Materials for Noise Reduction, and Thermal Insulation. Call 800 679-8511. | Second Skin Audio, Made in the USA. but it will depend on your budget. I say go for the Stinger because it's a great price for what it is.
The 5 volt preout will only come out once you turn up your volume close to max. That's why you should set your gains with about 3/4 volume or higher if it's a clean signal. I believe the 80prs is clean 65/65 from what I've seen so you should have no problem using most of it. You should reset your gains doing it this way. It'll be way cleaner