Well that sub is a dual 4 so it could only be a 2 or 8 ohm load. I am assuming that its a 2 ohm load. In that case there seems to be a few problems.

1. Is that the px600.2 is actually a 2 channel on jbls site. Are you sure its not the px300.4 or something?

2. Is that the px600.2 doenst look like its rated down to 2 ohms so that could be part of the protblem. Also is this just a recent problem?

Have you used the px600.2 before and it sounded fine? The reason I ask is that it looks like it has the dbo circuit and one guy found out that it will cut the low end no matter where it is set and there is no way to defeat it. Maybe try adjusting that to see if it helps. If not maybe you can borrow another amp and try it on the sub? Where are your crossovers set at?