Do Streamer only devices really impact sound quality?

I would definitely agree that the ethernet stage can be the most complex, and certainly seems to allow the most possible upgrades, warranted or not.

And I would certainly agree with @sns , that a decent switch can make things worse. I tried the Network Acoustics Muon between my Netgear Orbi router and my Zenith streamer and liked it a lot, until I experienced severe drop-outs (the NA guys suspected the router wasn’t very good at “auto-negotiating down to 100mb/s”). On their advice I added a bog-standard TP-Link switch with decent iFi power supply between the router and streamer. The drop-outs disappeard but it sounded awful - really brittle and bright.

I currently have an SotM ISOCAT7 and dCBL-CAT7u.

Lost my chain of thought here. I agree harshness, sense of hardness can be attributed to streaming chain and/or system. So, assuming one has rid themselves of this and still has fatigue. This is very likely fatigue due to jitter. I eliminated vast majority of harshness/hardness some years ago, eliminated minute residual over the years through streaming and system tuning. Still remained this slightest sense of fatigue,  like excess precision, a bit analytical presentation, very hard to put finger on although I began to hear it as timing issues over time. New custom streamer goes in, great attention to noise and latency issues. Revelatory is only word I can apply to what I'm hearing, much greater sense of ease, I simply relax into music, its like the music has slowed down and come into its own natural flow, far more like the top tier vinyl setups I've heard. There has been substantial reduction in noise floor as well, but the newfound sense of ease is the revelatory thing. I attribute this to great reduction in jitter vs past streamers.

New custom streamer goes in, great attention to noise and latency issues. Revelatory is only word I can apply to what I'm hearing, much greater sense of ease, I simply relax into music, its like the music has slowed down and come into its own natural flow, far more like the top tier vinyl setups I've heard. There has been substantial reduction in noise floor as well, but the newfound sense of ease is the revelatory thing.

@sns  Very well said as to the incredible improvements that can be achieved by upgrading to better components in the streaming chain. 

Keep an open mind and audition some models. I don’t notice any difference with them and am content to play directly off the streaming stations. Put the money into speakers.

I made the following budget minded changes to the front end after initial  setup.  I have BS Node 2i and Topping D90 with AKM chip. Took some time research and learn.  Discussions such as this are very helpful.

1.  Better power cords for Node and D90.   $200

2. Hardwired ethernet cable to streamer. $40

3. Dedicated power circuit for audio system. $500

4.  Upgraded power supply to Node.  $130

5. COAX  from streamer to DAC. $40

6. DX Engineering EMI/RF filters on ethernet. $50

7. New cable modem router combination.  It included a LPS and eliminated  an older router and wall warts.  FREE

8. Herbies firm fat dots.  $80

Cumulatively,  these changes made nice changes.  Depth of soundstage,  transients, sense of space, details revealed in bass, etc.

Then I  added a Add-Powr Symphony IO.  This improved the sound even more.

Sure, each step could have cost more and would probably improve the sound even more.  If you upgrade the above steps, the cost goes from 100s to 1000s.