Here is what I am getting at. I am looking to add a new source as I think it is the weak link in my system. My system is all Meridian except for the source (HD621 -> G68 -> G57 & G55). The HD621 cleans up jitter (what jitter BW?) with the assertion that all digital signals become equal. Problem solved forever? Apparently not.
Sony PS3 and Apple TV are both connected to HD621 and PS3 sounds much better especially its bass. I am guessing the offenders are low frequency jitter and power supply noise making it through to the amp. In other words, HD621 does not clean up as well as desired.
If the processor does not clean up the source adequately then the source itself must be cleaned up. Hence, I conclude a newer cleaner source is in order. The question becomes, does it have to be analog or could it be digital? For integration I would prefer the source to be digital like a network music player.
So to answer your question Kal, "Why indeed?", the answer may be because a good DAC output is likely to be cleaner than a good digital output. But I am not positive about this. Perhaps skipping over ATV, MAC mini, and the likes and going to a network music player with a top notch power supply would do the trick. If I had a spectrum analyzer I could quantify the answer rather than just suppose.