One percent is generally considered the limit of human perception. Below this, distortion is inaudible to nearly everyone. This occurs when different frequencies modulate each other and produce unwanted artifacts not present in the original sound. Higher and lower frequency response is not necessarily better. If stated, this figure should be less than plus or minus 3 dB. this will assure a high degree of linearity.
CD players have extremely low noise inherent in their technology.
If the radio does not have a low-level output, you will need to use the high level, or speaker output. Generally, this is less desirable because of the noticeable increase in distortion and noise in such an arrangement. If your amp is for bass only, this may be a less significant problem. Now turn on the radio. 12 volts should go to the lead and turn on the amplifier.