Here is how you figure it out....Without a meter. 10 minute troubleshoot. Do each test, and put everything back to the way it was when you started between each step.

1st switch your amp from stereo to mono, did the volume of the low playing speaker change? Switch back to stereo.

A)if yes, skip to 3. If no, go to step 2

2.Switch your speaker wires from the amp from the left channel of the amp to the right and right to left. See if the problem follows the swap or if the speaker still plays low.

A)If the same speaker still plays low, bad speaker or short in wire from amp to speaker. If the swap makes the other speaker play low, it's either the amp or head unit.

3. Try swapping the rcas at the amp from left to right the same way you did the speaker wires. Diagnose the same way as in 2.

A) If the problem follows the swap and switches which speaker is playing low, you have a problem between the head unit and the amp. Bad head unit, rca output, or short in the rca.

If after you swapped everything above, and always get a low output on that channel of the amp, you may have a setting on the amp set wrong, or a bad amp.