I do not get the obsession of where the gain control on the sub is (1/4, 1/2, 3/4). The important part is cablibrating the sub properly between the sub and receiver and if it is the case it is at 1/4 or 3/4 it really doesn't matter as long as you are not over driving the sub in distortion. SPL meters are good investment for calibtrating your sub properly
I do not recall you ever mentioning the subwoofer setting you have set for your EP500 on your receiver which controls the voltage/trim being sent to the sub which will effect the sensitivity of the gain knob on the 500 and the sensitivity of the auto/on feature (if it has one).
Are you still calibrating by ear?
