How can different CAT5/6 cables affect sound.

@retiredaudioguy 

Perhaps I am lucky (or hard of hearing) but yesterday I listened back to back to the Weilerstein Elgar from my Aurender SSD, USB connected to the Esoteric K-01XD SE and the same work streamed from Presto through the Bluesound Vault, TOSlinked to the DAC and detected no differences. 

help me understand how this comparison is relevant in a context of Ethernet cable discussion. How’s it even valid at all? 
You’ve heard two totally different streamers (drastically different design) connected to the same DAC using different interface methods, different source and most likely two completely different versions of the master file. I mean no disrespect but it does appear as if you’re hard of hearing. Or there are other factors at play here that negated all possible differences, i.e. room acoustics, setup, circumstances under which this evaluation was conducted (room full of people chattering, windows open with outside noise interfering with critical listening, etc.)

I think it is relevant in that the streaming signal was received through a cheapo Cat 5 cable, an inexpensive streamer and plastic optical link, the other signal was USB from data stored on the Aurender.

I believe that Presto's streaming service aims to provide the same quality of sound as their downloads.

My thesis is that, with a truly well engineered DAC (e.g. The K-01XD with an external rubidium clock), how the digital data gets there is not relevant - given source gear that gets the correct data to the DAC, which all minimally competent equipment and cables do.

Or, perhaps, Presto's MPEG4-SLS codec is more resilient than Qobuz's FLAC, I do not know.  Or perhaps my hearing has gone.

I have been researching the issues and the MPEG4-SLS codec will scale back the quality of the streamed data if there are bandwidth problems.  Those problems could arise if the streaming device detects data errors and causes retransmissions to be necessary, and that could be caused by a poor Cat 5 cable.

Remember my initial question was "how can Cat5/6 cables sound different?", I was not asserting that nobody actually hears a difference.  I guess that the answer could be that the streaming service downgrades their transmission if there are too many data errors.

@retiredaudioguy I am not here to argue with you but it absolutely matters how digital signal gets to the dac. Toslink in my experience is inferior to spdif or USB all things being equal. Different units own clock priority. Digital cables do matter, including Ethernet ones. Even with your Esoteric DAC. 
In addition, your Aurender and BS streamers each have proprietary DSP with Aurender caching the stream onto SSD prior to sending it out to the DAC and BS buffering the stream. Add to that the fact that you don’t know the master files being used from presto and SSD. 
But then again you heard no difference on top of all this with Flac from Aurender vs Presto stream thru BS. Consider yourself lucky. 

To add…if I am not mistaken, toslink will result in synchronous clocking meaning your DAC clock will synchronize with the digital signal that’s output of your streamer or transport. Same goes for SPDIF. So whatever jitter is passed from BS streamer clock to DAC will impact the final result. Even with Esoteric. 
With USB, the process is asynchronous, meaning the DAC’s clock, in your case Esoteric, is going to get the priority. I’m pretty certain your DAC has a clock that is superior to both BS and Aurender if the latter is a lower model without OCXO clock.

You can’t bombard your DAC with jitter and expect the same results as you would get with a clean signal.
It isn’t all simple and there are factors both internal and external that will affect the sound. 

As to Ethernet cables, I’ve tried several and they all work but don’t sound the same. 
I’d say Ethernet cable is probably at the bottom of the list with components and room acoustics as well as clean power well above network crap. 

Toslink in my experience is inferior to spdif or USB

@audphile1 

TOSLink and USB are connection systems, whereas S/PDIF Is a communications protocol.

S/PDIF was developed in the early 80s, specifically to support the then-new CD format. It was never revised in the ensuing half-century (almost). It does not support either hi-res PCM or DSD. It can safely be considered obsolete at this point.