Updated measurements at 0 and +10:

amp.....Vac.....Z......Power.....SPL
-30......0.9....8.5......0.1.........75
-20..... 2.5....8.5......0.7.........84
-10..... 9.5....8.5......11..........92
..0.......30.....8.5......106.........95
+10......37.....???......???.........???

At 0, I still had what sounded to me like a pure tone. At +10, my amp was distorting badly. It's hard to say what frequencies exactly were going into the speaker. So it's meaningless to attempt to do any calculations with the gain at +10.