From the disk to the input of the DA converter, it's just reading data from the disk and transferring it. The way two drives would differ would be in read errors. Specifically (it seems to me, anyway):
- the frequency of errors at the outset: basically how good the hardware (laser, etc.) is at reading the disk without missing or misplacing a bit;
- the ability to correct these errors and still get good data in the time required for real-time playback: buffer-size, error-correction algorithm, speed with which the processor runs the algorithm, etc.;
- how well it deals with an error that can't be fixed in time: interpolation algorithm and processing of same.
Do DVD/CD players, as a class, generally differ from CD-only players on these bases, when playing a CD? I don't know. Someone with considerably more technical expertise than I have might. My only vague observations would be:
- pro-DVD player: they're capable of reading DVDs, which have much denser data on the disk than CDs (about 6 times more), so presumably they generally have more precise laser mechanisms; also they've got to be able to play movies in real time, so they maybe have more buffering and processing power (though I'm not sure about that, as the datastream for a compressed movie may not be all that much more dense than CD audio, and the people who spec design requirements may figure movie playback is relatively tolerant of uncorrected errors).
- pro-CD player: everything is designed and optimized for reading CDs at exactly the data rate that CDs run at.
Those may be the "various technical reasons" GuitarLegend has suspicions about, or there may be some more. I'm not super-convinced by my own observations, though maybe a bit more than I would be by the salesman's spiel.
From there to the line out, it's doing an audio DA conversion and running through a bit of analog circuitry. The DA converter and analog path certainly could differ from one machine to another, but I don't know if one could generalize between a DVD/CD player and a CD-only player. I suppose if they're both sold at the same price, you could figure:
- the CD player is better, since all the manufacturer's cost went to components and capabilities useful for audio, rather than stuff that you're not using when you play a CD in it; or
- the manufacturer just markets them differently, and makes a better margin on the CD players.
Whether any of this makes enough difference to matter is a whole other question. When compared to speakers? Probably not so much.
