One thing that comes to mind is dirty switches. Do you hear "crackling" when you turn the volume knob or when pushing any source buttons? If so, some contact cleaner may solve your issue. Another suggestion is if there is a mute or attenuation switch, use it and you'll be able to turn the volume knob up more. If your CD player has a gain control you could try cutting back the input signal which would allow you to turn the amp volume up as well.
