We say source first, but what counts as a source?

It used to be clear, for the first twenty years of my hifi ownership, the source of the data was a silver disc, and the source of the noise was my CD player. Simple wasn't it? Now I mainly stream from Qobuz via a PC, so the source of the data for music I listen to these days is...well, I don't know where it is. I suspect it could be a data centre in France, but tracing it could be a challenge akin to finding the source of the Nile. With the data whereabouts unknown and that accepted, what counts as the source in hifi terms? The PC, the software running on the PC (definitely important!), the ADSL router, the exchange, a data centre or NAS drive, where does it end?

This question entered my head after listening for a while the other day, and thinking afterwards how wonderful the DAC-V1 and SN2 combination was working. In my mind I had thought of the DAC as the source, as everything beyond it was non-hifi, but then started wondering, is it really? As I read about companies producing audiophile NAS drives, with audio grade SSDs in them, and people worrying about power supplys to network switches, audio grade ethernet cables and auralics, I start to wonder if I am missing something.

I've been of the mind that with most DACs being electrically isolated from the incoming digital signal, and with clever buffering and re-clocking, which I believe would intervene regardless of digital source, the audio stream coming from the USB bus of a PC, should be the same, and as good, as any other streamer in to the same DAC.

Thinking about this topic has, once again, left me confused, though that's not difficult. I would be really interested to hear views on the importance of various elements in the digital domain, when streaming from a NAS, or online, and prior to reaching something like a Hugo, or DAC-V1 etc.  

Thanks