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.
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Enjoy the music, not the equipment.