Is the imbalance something new? Have you changed speaker positions, rearranged the furniture in the room, or made any other changes that might affect how the left and right speakers separatedly interact with the room? (One can always try headphones and see if the channel imbalance is still present.)
Another inexpensive test you can run yourself is to get a voltmeter -- plenty of choices for around $30 or less. Then find and download a monaural 1 KHz continuous test tone - mp3 is fine for this. Either burn to a CD or add to your streamer collection and play from there At either the preamp outputs or speaker outputs, check the voltage -- it should be identical for both channels. This will help give you an idea as to whether you have a problem in one of your component's circuits or an acoustic problem in the room.
Good luck!