Sorry, been sorta out of the loop due to a car pulling in front of my bike and other issues. I paid $450 for a single pair. I was offered by a dealer I'd rather not name $300 for a pair, but when I mention that price to Hertz dealers in San Diego, they get extremely angry. I don't want to burn my source here
I assume you mean sealing the subthump speaker adapters, and not the adapters for the amp electrical connections. The ST speaker adapters are particle board which you then attach with sheet metal, or less-than ideal sheetrock (drywall) screws to a metal surface. Two non-forming, hard objects, one of which is oscillating (making sound) at multiple freq's usually need something between to buffer and make the speakers "sound" better. It's also to provide a better performance, as any sound waves that come out of the back that are able to immediately come "around" the speaker to the front will cancel out. For an obvious example, turn on a heavy bass song with your sub in a box. Now take it out, and do the same. Obvious difference, but the principle is the same with sealing a mid-range driver. Tweeters are in a sealed enclosure usually, so this is why it's an issue mostly for the mid-drivers. Make sense? I sorta gave a long answer.