Getting different output levels from left and right channels. Ideas?

[Atlanta007 10+ year member]

I've been running an alpine cda 117 head unit for a while, without an external amplifier, to 4 speakers. The sound was not great, but it worked and I didn't notice any issue as far as one channel having more output than the other, with balance set at dead center.

I recently added an alpine pdx-F4 amplifier. I ditched the original door speakers and I'm running the amp bridged to 2 new door door speakers. I turned the internal amp off so the original rear speakers are not on.

With the gains on channels 1&2 and 3&4 of the F4 set to the same level, I get quite a bit more output (higher volume) on the left channel than right. When I reverse the speaker plugs I get more output from the right than the left. I've double checked my wiring (polarity, etc.) and can't see anything wrong.

Any ideas on how to determine why this is happening? I don't have an oscilloscope. I guess I could adjust the gain on the right channel higher than the left, but it's a good bit different, and I'd like to diagnose the issue if possible.

Thanks for any input.

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