I am trying to decide on a new single-DIN HU. Almost all of the ones with the features I am looking for are budget models from manufacturers like BOSS, Xtronics, etc, and so I fear are likely to sound pretty bad. On this thread though there is a good point about much of the distortion being caused by under-powered amps in single din units.

This makes me wonder, if the analog signal produced by the top brands & models really superior to the cheap ones?

After all, the audio sources are digital. The decoders all work the same way. Yes the expensive units are likely to have higher bit DACs than the cheaper, but the same range of frequencies should be able to be hit even if the resolution isn't as good, right?
If you were to take the (preamp) line out of a cheap HU and connect it to a top-of-the-line amplifier, would it sound that different from the same setup but using an expensive HU?