a simple test to see if the problem is the bluetooth, take the outputs of the bluetooth RCA plugs and swap them to the amplifier (move left to right, and the right one to the left side)

if the problem moves to the other speaker, the problem is the bluetooth, if it stays in the same speaker proceed to the below test. 

a simple test to see if the problem is the amplifier or the speaker is to

take the left channel output and hook it to right speaker 
take the right channel output and hook it to the left speaker 
if the problem moves to the other speaker, then the amp is bad 
if the problem stays in the same speaker, then the speaker is bad