I am not really good in the audio system. I am just beginner.What do you mean by source?I will try to go get a set of headphone to try what you suggeted.
Examples of source would be CD player, FM, etc. Where ever the audio signal comes from! If you listen on headphones, that by-passes the amplifier in your receiver and the speakers.
If the headphones sound good, you would know the problem is with either the amp or the speakers.
If the headphones sound similar to the speakers, then you would know the problem is either with the source or with the receiver before the amplifier.
If your source is a CD player, many have headphone outputs so you can plug into the CD player and that will determine if the problem is at the CD player or with the receiver.
Basically, we are using the headphones to test at different points along the signal chain to see where things go bad.