We were having problems with the spe...

You need to think about what happened between the time it seemed to be working and when it started to be 'different'.

Did you move it, disconnect/reconnect something? Also, please reword the problem description: "the left and right speakers had to be plugged into different plugs (B & C) and >>> oth in the left of those <<<." What's it mean?

Also, think about if the problem exists with all or just some sources. FM on the receiver is the easiest to troubleshoot with because there are no cables between it and the amps, hence very little chance for operator error.

Is there a speaker selection setting that you may have overlooked? (Second knob from the left?)

The better the questions, the better the answers.