How to get into high end digital? (Feeding a DAC)

The only explanation I can offer that will be from a subjective viewpoint, is that when experiencing certain cable types within the System there were presentations that were immediately recognized as being able to be a detractor and not an attractor.
From recollection the most short lived cable used in a trial was a Silver Wired Cable.
In contrast a Silver Wired Din > RCA Phono Cable has been in use for more than Three Years, and has only very recently been changed, this cable has been compared to approx' Six cables over this period, that have been made up of varying purchase values.
The retained Silver Wire Cable proved to be a satisfactory cable throughout the usage period.
Its replacement is a PC Triple C Wire Cable.  

Just to add to what others have said, streamers/digital sources all sound different. I am on my third streamer, the other two are okay, this one is better, all through the same DAC. I will keep looking. I have one of those Nakamichi 5 CD changers and put it in my main rig once. It has no digital out, so you get the digital source and DAC in one. Almost unlistenable compared to streamed versions of of the same ripped CD's through a streamer. Anything you do will be an upgrade there. I have gone completely digital with all my CD's ripped on a NAS, no plans to go back to a CD player, however I understand why some folks like/want them. But understand, even if you are using an external DAC, the digital stream coming out of the CD player can affect the sound like any streamer can. Everything matters if you have the ears to hear it.

The Ygg is a good choice, I have a Gumby and am very happy with it.

Disregarding for the moment the issue of transport and cable, there's quite a price differential between the two DACs you mention in the original post.  I'd recommend you take a hard look at the Gustard X26 Pro.

What is the budget?  There is a 2K price differential between the Topping and and Yggdrasil.
…
  If otoh, the system can detect subtle differences between components, get back to us with a budget

Well I picked those two as what I am looking at.

• The 10k$ Merging+NADAC is beyond what I would be comfortable budgeting
  
• And I would rather not spend anything if it is a flaw endeavour… so if it requires 20k$ to get something as good as vinyl, I would skip it, and just go buy some vinyl, and some bottles of Cab/Pinot/etc/…
  

One huge question for you, is how tied in are you to your silver discs? Many of us have reported improved SQ playing ripped files from a NAS that sends the files to your dac through a network player/streamer. This is a whole convo in itself, but I would suggest considering using any computer drive to rip discs to a NAS. 

• Bingo. I need them in a car, and I like having them rather than a download.
  
• If all the transports are flawed, then maybe I should just chuck a car audio CD player into the system to play physical CD, and go NAS for the good stuff? The car CD player I have is supposedly pretty good for the CD transport and DAC end.
  

If your new "Home Theatre pre, which is Roon capable" means it's a Roon Ready endpoint device, then it can be your network streamer/player. Just connect it via ethernet cable to your network. Run Roon Core(the main Roon software) on 1) An Intel NUC if you're into some DIY-ish assembly 2) Small Green Computer/Sonore Sonic Orbiter 3) Roon Nucleus or 4) Mac or PC.

• I am already running a NUC for the NetFlix streamer, and another for Linux so I am familiar with them. (& thanks for the links - I’ll give them a whirl)
  
• I guess I come out of the NUC on one of the HMDI ports? And into the HT-AVR/Pre?
  

Thanks @dabel@ctsooner  Thanks- I’ll PMThanks all