As I was reading the replies in this thread, I was thinking that the probability that the speakers are bad was going to be low. It's hard to destroy speaker drivers - you have to try hard quite with destruction on your mind. It's possible to do but not so easy.
So, it could be
* a failing speaker x-over component
* a cold solder joint in the speaker x-over
* dirty RCA contact all around + dirty speaker contacts
* failing power supply electrolytic capacitors in the power amp
There might be other potential causes but these are the ones I can think off immediately....