I can think of only a few things that might offer better SQ in terms of the network infrastructure used to deliver content to ones audio system.Good post but I think that having IT experience here could be a partial handicap in a way if it prevents you from trying these network based tweaks. Noise is a huge deal in a system and many things can generate it. But I also think you make a good point about wall warts. They can contribute a lot of noise in my experience.
The great thing about your background is that it could help us focus in on more likely culprits.
I continue to have new experiences that show that everything in the chain matters.
1. The use of an optical break (ie..Fiber, Fiber media converter) immediately after the ISP modem. Basically sitting between the main WAN Port on your Firewall/Router and the ISP modem. I do this today, but not really for any SQ related reasons. My main reason is to shield the Internal network equipment and Internal power grid from surges that can enter over the coax cable leading into the house (I use cable internet but DSL would be the same story). As a secondary reason, this can provide an an electrical break from outside world noise found on this line. There are obviously plenty of other ways for outside world noise to enter ones home but eliminating even one of those many paths for little cash outlay and hassle is hard to deny or ignore.
2. If you can get away with it, depending on the audio equipment being used, I think not using any switch at all between your music server and the DAC is a better choice than using an "Audiophile Switch". This would assume the DAC offers an Ethernet Input of course. I'm talking about having a "Home Run" Ethernet connection from a music server having more than one NIC card to the DAC directly. I do this today in my own audio system.
NIC #1 connects to your main switch/home network
NIC #2 connects directly to the DAC via the "Home Run" Ethernet connection
NIC #2 would utilize APIPA (Automatic Private IP Addressing) to supply an IP Address for its own NIC and also it would provide the IP Address for the DAC (assuming the endpoint is set for DHCP). Both would end up with a 169.254.x.x IP address and can speak to each other directly.
The way I see it, at the end of the day, it comes down to how well implemented the Ethernet Input is on the DAC side. If its sensitive to electrical noise propagation over the Ethernet cable then the DAC has other problems these tweaks are compensating for but in terms of the data stream itself (ie..the 1's & 0's) its impossible (as far as I'm aware) for this feared noise/grunge to inject itself into the data stream without also corrupting or changing the 1's & 0's that make up that original content. In that case, the stream just wouldn't work or it would be painfully obvious that something is wrong.
So IMO, unless there is a pre-existing issue in the system, the use of an "Audiophile Switch" or high-end Ethernet cable offers little benefit.