The dial is for gain. Axiom cannot predict in advance how strong/weak your source signal will be. The dial assumes the worst. Your receiver is not the worst. Your receiver + EP500 at 20% propabaly leads to 500WATTS peaks. A weaks source + EP500 at max equals 500WATTS. It's all relative.
You need to get an SPL meter and measure the actual level you need to set your sub to match your speakers and your receiver's configuration. There are no pre-determined answers, except that it is unlikely that your sub needs to be more than 20-30% with modern days equipment.
