I think I might have figured out the all clear bonus. I just got a single +1 combo all clear on level 12 and got 11400 points, which would work out as 1200 for the line clear, 600 for the +1 combo, and 8*1200=9600 as the all clear bonus. Assuming I was actually on level 9 instead of level 8 when I got the 8550 (I was probably looking at the 80-some lines cleared instead of the level number), then that would be 900 for the line clear, 450 for the combo, and 8*900=7200 for the all clear. So apparently the bonus is 8 times the points of the line clear; however, I think when I got an all clear with a double I did not get that many points, so perhaps the bonus is only 800 times the level number rather than 8 times the number of points for the line clear. I'd check, but for some reason the all clear I always seem to get when I'm playing around is the single with the horizontal I.