I don't think it has a lot to do with old DACs; problem was (is) CD's large dynamic range and that above 0dB you go into digital clipping which sounds horrible; no "gentle" distortion here; above 0dB is sudden and serious distortion zone.

Subjectively, a regular tuner sounds as loud -and depending on station*- or even louder than a CD player on an identical line-level input in a regular system. This is because of its limited dynamic range and capability to exceed 0dB once and a while without one really noticing it.

Nominal level on most CDs is often at -20dB or more to allow for high, short transients; this brings subjective level in line with more traditional line-level sources.

These high transients are also the main reason why often level normalisation of MP3 files still doesn't give the desired result and you keep fiddling the volume control going from one song to the next, particularly with decent recorded orchestral music, by the way.

*: Many radio station heavily compress their broadcasts specifically with in-car listening in mind. In the UK, worst (or best, depending on point of view) example has to be Classic FM which makes liberal use of Optimods...

Oh well, thought you might wanna know...

Cas