How can I improve SQ of my Dell desktop/Roon set up?

I just installed Roon and the streamer/DAC Teac NT-505.
Here is my system configuration: Roon core runs on Dell I7 with music stored on SSD, 40 feet router extender from office to my HT room. then another 35ft ethernet connection from the router extender to the Teac 505. I have a dedicated high end SACD/CD player connect via Coax to the Teac. I have performed the following tests/results using a same source music:

  1. CD player running stand alone. Most transparent, but not as expansive sound stage as #2, SQ is about 80% of test #2, this could be the DAC in my Cd player is dated in comparison to the new AKM DAC chips in Teac.
  2. CD player/Coax cable/Teac DAC. Best sound. more open sound stage, better dynamic, but not as transparent as in #1, could be due to the inferior coax RCA.
  3. Dell NAS/Roon/Ethernet/Teac using Iphone/Ipad as remote. Open sound stage, warmer, but not as good air around instrument as in #2.
  4. Lumin app/Teac/USB flash drive using Iphone as remote. Better sound than option #3, but not as good as option #2.

To my ears, SQ of option #3 is decent but not as good as other options. How can I improve SQ of Roon/PC setup in the #3 test?

May be I should get a better setup like Roon Nucleus, or streamer and separate DAC? or interface to the Teac via USB/laptop and just use the Lumin app?
I have about 1000 albums stored on NAS, and don’t currently use any streaming services.
Thanks for any input.

I'm thinking your bottle neck is your computer. you may want to try a USB isolation device as an inexpensive option to see if that helps. I know of one off the top of my head from Schiit Audio there are many others though. USB in USB out, USB in coax out etc. Streamer may help to as its dedicated to the task. computer power supplies are dirty buggers. 

Anyway I'm no way a digital audio guru just info I have gleened the last few month as I have built up my digital front end. 
Get an Ethernet-2-USB converter from Sonare, such as a microRendu and you are miles ahead. I have a similar setup on a very internally busy computer and the rendu chokes of that computer noise. Now there are even more options you can consider, such as using an optical network that will eliminate the noise on the Ethernet too. Take a look at the following link for some info on this.