half broke amp/ weak channel

Can a amp partially break? Can an amp work but just not to its full potential as in can something or part of an amp burn out and the amp continue to function but only perform about half as well as it should or do amps either work 100% or not at all. I ask because I have a pioneer head unit with 4 volt outs. I have it connected to an alpine 4 ch amp. The gains on the amp according to the manual are 4 volt at minimum gain and .2 at maximum gain. If i turn the gains to minimum I can barely hear any sound, as in it would be louder hooked straight to the amp built in to the head unit. If I set both gains at the .5 volt mark about two thirds the way up it sounds "OK" but still not like it should and I have fade the HU about a third to the front to make the front as loud as the rear, and they are the same speakers front and rear. To get good but still not correct sound I have front gain all the way to max and the rear gain at about 3/4. This however still does not let the sound get as loud at the same exact speakers in my sons car and his car also has an alpine amp that is ten watts per channel less with the gains less than half way up.

Do you have severe engine noise when the motor is running? Does it make a popping sound when you turn the volume from 0 to 1 and vice versa?