Do external clocks make a difference?
The answer is yes. How much? Well, we can measure the difference. Whether one can hear that or not, is another story.
Without going into too much extraneous gibberish, we have the use of something called a Wavecrest DTS-2077. It allows us to measure the amount of jitter on a signal. Its strong point is that it can separate jitter into its two main components: random and deterministic.
Random jitter is just what it is called. It is Gaussian in nature, and exists in everything.
Deterministic jitter is a bit more involved. To simplify things a bit, we can say it is predominantly correlated jitter. That is, jitter that bears a strong resemblance to the data stream itself.
In the Squeezebox products, (which are very good, especially for the price), the designers have to make certain compromises, to meet the price/performance goal.. (All products do, but ones at the low price end have to do more of it.) The main compromise is using a hex inverter, to function as 2 clocks, and the SPDIF driver stage.
So, having said all of that, one would expect to see deterministic jitter, and yes, it does exist. It is a tad higher than the random jitter.
Ok, the question you all want to know:
Does changing the external supply affect any of this?
Yes, it lowers them. Not a great deal, but we can measure a difference. Can you hear the difference? Probably. Maybe. I dunno. It depends. All very valid answers.
The other question you want to know:
If one separates the clock and SPDIF functions, totally, what happens to the SPDIF signal?
If you do it properly, the deterministic jitter goes completely away. The random jitter can be much lower, depending on the quality of the clock.
Hope this sheds some insight.
Pat