Power supply to modem make a difference?

Good streamers completely isolate the signal electrically and cache the bits. So, it does not matter how good or bad your network is. I run my Aurender streamers from extenders. Changes in extenders, cables and ethernet regenerators have no effect on the sound quality. A red book CD, stored file or streaming sound exactly the same... as does vinyl. 

Cheap streamers can achieve better by improving the whole network... but it requires improving stuff that is only important for your audio system and becoming an IT guy. Much better just  to purchase a good quality streamer.

I added an LPS to my main  xfinity modem/router which is located in another room from my main system.  Also, I added another one on my mesh router that feeds my main system’s streamer.   My streamer also has an Lps built into the unit.  Did the Lps’s make a SQ difference?  I don’t know, I did not do blind A/B comparisons and have no plans to.  It certainly didn’t make things worse.  My goal was to reduce noise in the A/C lines.  When I added an Lps to a prior streamer (ifi Zen Stream) it made a noticeable SQ improvement.  Really depends on the device and what you want the Lps to address. 

@ghdprentice  What do you mean by isolate the signal?

I use a Roon Nucleus with Teddy Pardo LPS. 

It depends on the quality of your streamer. Good quality streamers isolate the signal from the modem/router or extender. Cheap ones do not. 

The issue IMHO isn’t noise via network but noise via the AC line, and potential for surges coming in from the cable line unless it’s fiber already.

For power supply issues I use iFi supplies like these. 

For surges I have one of these outside and then use Ethernet to fiber adapters to put a 1 meter air gap between the cable modem and my router.  I live in a lightning prone area where lightning did enter several neighbors homes the first year I was here, so I'm more nervous than most.

Mine powered by LPS, more about not contaminating dedicated audio circuit with SMPS. This alone is good enough reason for me. Also, Broadcom chips reportedly and measurably best for modems, don't know if that still holds true in every case.

I would think in the same way it makes a difference for any other digital component such as a streamer or a DAC. 

No. How could it make a sonic impact?