It sounds like others have given their opinion of the factory CD player, but they didn't give you much of an answer to your question.
The factory CD player needs to be used with the factory amp. The factory amp was used with vehicles with premium sound with cassette players as well. Not all factory systems with cassete players were premium sound with amps. All factory CD players had premium sound with amps.
My SVT came with a factory CD and I swaped it out for a factory cassette player within a few days of purchase. I obtained a cassette player ment for use with the premium sound amp.
If someone tries to install your CD player on a car without the amp, the connectors will not work and if they did find a way to make it work it would not be very loud with poor sound quality.
