So either an amp is defective, a speaker is defective or there's something up the chain.
Switch the speakers. Does the problem follow the speaker? Is the affected speaker low when plugged in to the left front channel, for example? Then the speaker's the problem. Fix or replace.
If the problem doesn't follow the speaker, switch the inputs from the player to the receiver. Does the problem follow the switched location, or stay in the same receiver channel. If it moves, it's a problem with the player. If it stays in the same place its a problem with the receiver. Whichever one is broken needs somebody to fix it. Or replace.