Sub turning off, co-worker's dilemma

[allstar780]

A co-worker of mine asked me about an issue she's been having... basically she has a denon receiver (doesnt have the model number, will update when she gives me it) and a sub she said only has speaker wire inputs (she doesnt know anything about it... obviously it's some type of passive sub... will update) and basically the sub output on the receiver is shutting off at a certain volume (i'm assuming that the amp doesnt have the power to drive the sub or there is some sort of short)

basically I need suggestions for what to try if she drags me to her house to try to fix it... obviously i'll check and re-check all connections, what else?