If the sub still hums with only the eletrical power feed plugged into the wall then it is the amp and a call to Axiom should be done ASAP. I would hope they still cover it if it is just 2 months out of warranty or at least are very generous with costs.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70