This isn't a high-fidelity forum. Get a good book that explains hi-fi and this will start to make a lot more sense.

Basically, you start out with all the music sounds in the studio.

• Some is lost during recording
• More is lost during media production (CD, Tape, MPG files)

When played

• More is lost in playback (yes, even CD is not "perfect")

• (If Radio) More is lost in broadcast (remember, they use gear to reproduce the signal before broadcast, and radio signal seriously reduces the frequencies that are reproduced)

• More is lost in pre-amp (head unit)
• More is lost in power amp (head unit and external unit)
• More is lost in speakers
• More is lost in listening environment
• More is lost in ear (not all people have perfect hearing)

So by swapping head units, you affect the part involving playback (radio or CD) and amplification (pre-amp, head unit power amp)

That's a big part of the formula. Get a book and do some reading!