Speaker balance problem

I am very inexperienced with audio technology so please bear with me. I had someone set me up with a Sonos system (fabulous). It is connected to a Yahama RX-V657. The receiver output is zone 2 which connects to a Niles SS-6 speaker selector system. Everything is great except for the speaker balance on all 6 sets of speakers. The left channel is ok but the right volume output is much lower. I tried the Yamaha manual setup to balance the speakers to no avail. I also made sure the Sonos controller was balanced properly. Anyone out there to help me?

it is either the receiver or the wires somewhere.

First thing is to switch the channels on the back to see it is from the receiver or from the cables. Switch left with right. Hear and you will find where the problem is.