Low volume in factory GMC pickup radio

The other day the battery went dead in my 1991 GMC pickup truck. I purchased a new one, and since then, my radio is not playing as loud as it used to, and there is no bottom end.  When I turn the power on to the radio, I don't hear the initial little "pop" through the speakers like I used to.  I reset my equalizer to where it was prior to the dead battery, but it made no difference.   Where would I begin troubleshooting this?

Thanks,

Bob