I went from a 6" subwoofer in an amplified tube with a 100 watt amp inside to this bad mutha:
Digital Designs 12" Subwoofer powered by:
Power Acoustik Reaper 2000 watt Amplifier
Then came the issue of it sliding around. I purchased 4x 3" corner braces, drilled holes through the (hollow, as it turns out) cargo shelf, used 2" long #10 machine screws, then secured the other size with 4x drilled out electrical panel backings, some washers, and nylon locking nuts. It's now very snug. And it bumps. Oh lawdy does it bump. The downside: I lost about 75% of my trunk space. Worth it.
Finished product: