It is far easier to reproduce and maintain the quality of a digital source, such as a CD. But once the signal is converted to analogue it is much harder to maintain this quality, with the biggest issues being the introduction of noise and the diminishment of both frequency range and dynamic range.
There won't be as big a difference between head units are they are primarily working with digital signals, whereas amps and speakers are working with analogue signals and there can be quite a difference between those built will higher quality components and those with cheaper ones.
Technically there are differences in the sampling quality of digital items and likewise the quality of the digital to analogue converter (DAC) but there shouldn't be any need for us to get that detailed (unless you want to
).Remember however, that regardless of how good your equipment is, like with any form of system, it will only be as good as the weakest / poorest quality item.
Many people also buy on looks and features and not just sound quality.
well then what about there speakers???