Hi guys wondering what input you might have on my problem?

I have two EP 600's daisy chained together. the first sub in the daisy chain has recently started popping randomly even when there is no signal input to it. I'm thinking it's an amp issue. I am using axiom cables to connect it to the Denon 4308 receiver. the downstream sub has no issues and works flawlessly.

I have had this setup for over 5 yrs and it has been nothing short of amazing until now. I was hoping you guys/gals would have some simple suggestions for me before I contact Axiom. anyone have similar problems?