Replaced front and rear door speakers and now sound volume has faded

I have replaced the front and rear speakers in my truck. Immediately after the install of the front and rear speakers I preformed a speaker loudness test fading the sound from the front to rear. All speakers had the same amount of sound intensity immediately after install. A few days later I disconnected the batter and now the rear speakers seem to be significantly lower sound intensity than the front. I have to fade the power to the rear speakers about 30-40% to get an even sound. This was not the case after initial install. I never played any music at the radios highest volume setting. So I am not sure how I could have blown the rear speakers already.