[Puma Cat]

Originally Posted by Xtraheadroom

So what is the largest advantage of using a high quality Ethernet cable?

Significantly improved audio quality.

Originally Posted by Xtraheadroom

I have about a 12’ run from my router to a pair of TP Link Ethernet Media converters (fiber) than back to Ethernet into my streamer/DAC. Could audible noise enter a poor quality cable here?

Yes.

Remember that the "digital signal" is actually an analog voltage square wave. The "integrity" of transmitting this voltage square wave is susceptible to a number of noise factors, all of which can impact audio quality in an audible way. Some of the noise factors that have an audible impact on audio quality are low- and high-source impedance leakage current, threshold and deterministic jietter, common-mode noise and phase noise.

Watch these videos by Hans Beekhuyzen:
https://youtu.be/B-StTplQZys
https://youtu.be/BbRF8z8dQFU

It's about switches, but the physics applies to Ethernet cables as well. Remember that when connecting Ethernet cables to a switch or FMC, it becomes a "system" and the physics has an impact on the system as a whole.

This video is informative as well:
https://youtu.be/bOKg1HJwUxE

Originally Posted by Xtraheadroom

Any recommendation would be greatly appreciated?

With respect to cables, back in 2019, I did a competitive benchmarking evaluation of Ethernet cables, including cables from AudioQuest, Belden, Wireworld, Supra, and Shunyata, and the even the "base" level Shunyata Venom Ethernet virtually smoked everything else. You also want to be sure of using an Ethernet cable that if shielded, has at least one end of the shield is "disconnected", so that high-source impedance leakage current cannot travel down the shield.

These days, I'd recommend using a Shunyata Venom-X Ethernet cable, which is quite a step up from Venom going from your router to the FMCs, and at least Venom-X or higher (Alpha, Sigma or Omega) from the last (downstream) FMC to your streamer/DAC. One of the impactful noise factors that none of the other Ethernet cable mfrs deal with is common-mode† noise, which is why if possible, I'd recommend using if its in your budget, a Shunyata Alpha level or higher Ethernet cable, as it has a C-Mode noise filter.

* – a square theoretically swings from OV (frepresenting a data bit of "0") to +2V (representing a "1"), but various noise factors impact the "squareness" of the square, resulting in phase noise, threshold jitter & determinisitc jitter, etc, which can impact timing.

†-Common-mode noise: https://techweb.rohm.com/knowledge/e.../01-s-emc/6899