Ok, you have not given any model numbers so I'll keep this nice and abstract.
digital info -> converted to analog signal -> piped to preamp
The "digital info" part is pretty much the same, modulo some considerations that I have a really, really hard time taking seriously. I might get flamed for this but it's getting really boring to reply to them.
The "converted to analog signal" is where real variations begin. Which DAC chip was used? How well was the circuit built around it? How about the DSP? Note that all the usual comments about the beauty of analog sound, avoiding silicon transistors, beautiful tube sound, etc. all bump up against this step here--as far as I know it is impossible to avoid using silicon transistors for this step. :)
Finally, the "piped to preamp" step, which is often neglected in reviews/previews/analysis of DACs. People sometimes go on and on and on about Wolfson this and Burr-Brown that, when it's this output stage here where we separate the men from the boys.
So going back to your original question, you'd have to look inside and see how each of those three devices implements each of the latter two stages. In principle, the DAC should do the best job, since its only function is to be a DAC! In practice, it depends on which models you are using.
I'll go out on a limb and claim that if the DAC does not sound better than the receiver or the CD player, then you either have a kickass receiver and CD player, or a mediocre DAC.
Unfortunately we do have a chain from source to speakers and if the preamp and amp and speakers are not up to snuff then the DAC's abilities may be unfairly obscured; so use the min-max principle, upgrade the weakest link in the chain and when you're sure that's the DAC, go for it and you won't be disappointed.
