Alright, it looks like this is the most comprehensive thing online about the RB1500. I contacted Randall, and they said that this guy wasn't made for very long, and said the closest documentation they have are the RB1000 and RB2000 manuals.
Looking at the RB2k, it seems pretty similar, except that there's no hi or low amp; it's all one unit. Because of that, all the hi and low splitting features are missing. Cosmetically, that appears to be all the difference.