DJ, there's really no need to try any further adjustments. If the volume control has no effect whatever, it's clearly inoperative. To clarify what it actually does, note that audio amplifiers have a fixed non-adjustable gain(typically increasing the incoming voltage about 30 times)and the control is a variable resistor which varies the amount of the voltage let in to be amplified from nearly 100% to almost zero. Obviously the control on your EP500 isn't doing this and appears to be letting in the full voltage from the sub pre-out. The amplifier itself is probably fine since it's driving the sub too loudly, but the volume control needs repair or replacement if possible. Contact Axiom.

Enjoy the music, not the equipment.