That's pretty impressive!
Back in 95-96 when I built my Mazda('82 626 Coupe), I could bounce(basically hover) a gallon milk jug 1/2 full of water off the top before I deadened it. It had 2 15's forward facing(fold down seats) in the trunk in about 7 cubes. I was competing in USAC 0 - 100 class. 91 watts of beastly power. The jug trick was at 42hz. My peak notes were 46 - 48hz(except on cool days which were around 44hz). Man those cars had some weak roofs back then. My lowest score was 141.?, Cause I left everything in SQ settings. Back then they played "Flashdance" for the legal SPL run.