Dakota Kid

Would anyone know possible reason for crackly/distorted sound coming out of my technics stereo system? I tested the speakers on another system and they were perfect. So its not the speakers. I connected a different pair of good speakers to it and their sound distorted also. The sound is fine until I turn the volume up past halfway. This happens when i play the tuner, cassette and cd's. They are all hooked into the amplifier so has to be the amplifier. Any ideas? I sprayed contact cleaner on all connections and also on the volume knob inside. Made no difference. Lastly the base and treble knobs dont seem to be working either. Maybe this is connected to the problem. Thank you for your help.