I think I know what the problem might be. The empeg has no definition of dimmer line active or not really. Rather it just stores two seperate dimmer settings for each state of the dimmer line. I would guess you have both set to 100% so it appears the dimmer isn't working.

You need to adjust the dimmer from the menu, while the dimmer line is actually active.

Just realised that that's what Tony linked to in the FAQ. Thought I'd post it anyway.