Have you tried running the sub directly to your reciever?
Assuming you have a receiver with preamp outputs.
Sorry if I am being presumptous...just trying to help.

It happened to me once and I had eventually realized that the preout on my receiver was bad.... Also try to bypass the xover on the sub and lower the volume to minimum...see what works at this point.

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Denon 4308
M80x2,VP150,EP600,4xQS8