Ok so I have seen this before but have not figured out how to fix the issue.
I personally ran speakers to my garage using Zone 2 to a volume control knob yet one speaker is producing 99% of the sound and the other speaker is producing a little sound, very little in fact.
Last night, I went to a clients house, and he has a whole home audio system with volume controls in each room, system is using Russound speaker selector in to a basic Audio Receiver. He is having the exact same issue, only one speaker is producing 99% of the sound while the other speaker is producing very little in each of his rooms.
Here is the troubleshooting I have done:
Rewired the ends from receiver
Rewired ends on volume control knobs
Rewired ends on speaker selector
Checked for cut speaker wire
Tried different speaker inputs on the receiver
Changed speakers to ensure speaker was not bad
I have heard of some sort of crossover issue but I am at a complete loss! Please help if anyone knows what to do.