Huh, wow. I just played "Clash on the Big Bridge" on Ultimate, got an SS, and then got... 5000+ EXP. How did I get so much EXP? That's the only time I've gotten anywhere near that much EXP from one song. I replayed the same song on the same difficulty again (even with the same characters), and only got 1200. Weird.
Speaking of, has anyone been able to figure out the metrics used to calculate how much EXP you get at the end of each song? I know you generally get more EXP as you go up in difficulty, but there does seem to be some random variance. Maybe it is just random?