You can mount a 10" subwoofer in the tray or a shallow mount underneath the tray. If you have any rattling from the bass, you can use some heat shrink tubing on the parts where the tray locks down. just remember if you decide to fold the tray up, that the size of the subwoofer box may interfere...
No welding, just drilled the new holes above the existing, reinstalled the rear coilovers and then cut off what was left below it. Shock skids go right back on. Just test fit skids to make sure they fit with your setup.
Installed and everything turned out great. Really makes a difference. Went with the napa leather. I helped putting the install guide together for them if anyone has any questions.