1) Check the speaker efficiencies. Doubling the power to a speaker provides a 3db increase in sound pressure. So a speaker with a 89 efficiency vs a 92db efficiency is going to sound a lot quieter. A 10 db increase sounds twice as loud.
2) Make sure they speakers are correctly mounted. The top of the speaker pushes out vibrations + , but as the speaker moves out, it also creates an equal negative sound wave off the bottom. These two sound waves can cancel one another out at certain frequencies.
3) Use the fader and balance to isolate each speaker and see how each sounds by itself.
4) Make sure you have the + and - connected correctly. If you have them reversed on one speaker, the sound waves can cancel one another out.
5) Check the speaker covers aren't smothering the sound, or touching the speaker.
