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

@holmz,
If playing from your spinning disc is the requirement, you'll need a transport or DVD player with a digital output(USB, BNC, dig coax, etc).  All DACs don't have the same complement of inputs, so be sure of compatibility.

SQ of DACs can vary quite widely from input-to-input. Some designers focus their design energy and invest in better parts for one or the other; it's worth considering user's experiences for the particular DAC you're getting. e.g. USB a decade ago was almost always noisier vs. coax & AES/EBU, but with much focus on improving USB due to it's popularity, newer designs have often overcome the concern.  

While Monoprice and other bargain cables can do the job, the impact of cable choice varies widely from system-to-system and is more distinguishable on more resolving gear. Advice from posters with no system posted or other similar context and explanation as to what they've heard for comparison, should be discounted accordingly. 

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.

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. Use any phone or tablet as a remote.
Roon is a game changing experience. I'd suggest this for a good visual on using Roon https://www.youtube.com/watch?v=Qe61rmLKc1E
and this if you're unclear on the 3 elements of Roon(Core/Endpoint/Remote) https://www.youtube.com/watch?v=qgoXpMFxi5g
Cheers,
Spencer 

"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?"

Cool. John Darko has a good video about setting up a NUC as Roon Core.  
No, you don't connect it to the Pre, just leave it near your router and add it via ethernet to a switch or the router itself. Your Roon Ready pre will be set up as the "Roon Endpoint", pulling the play queue from the NUC. You control it all via phone, tablet and perhaps from your HT pre depending on its features. Cheers,
Spencer

@lbarbish my post at the bottom of page one of this thread (last paragraph & the 2 youtube links) answers your question. If you watch and have followups, happy to help in your new thread. Cheers,
Spencer

XLD works great on Mac. Flac and AIFF are both fine. WAV does too but you will lose metadata when ripping (which you can add back, but extra steps.

@donniewn why suggesting that the Yggy USB will sound best from a computer? IMHE, USB-optimized DACs like Gumby & Yggy sound better with USB from a good streamer/renderer.

Audirvana+ is a barely supported hobby, and updates when they happen can be a nightmare. After 4 years of coping, I bailed and couldn't be happier about doing. Roon works SO much better and brings a far better experience. Cheers,

Spencer

Blue Jean ethernet cables are good too! 

@donniewn IME, even with a better USB card, a standard computer running iOS or Windows has so many processes running unrelated to optimizing audio. They also require more power (aka more noise). If you run linux stripped down to single purpose and a great linear power supply, that might be close to performance to a good renderer/streamer, but all that trouble is easily overcome with a purpose-built device...no extra processes, lower cleaner power, and ideally optical isolation. YMMV.

IME, Audirvana+  updates corrupted my library and required finding hidden files and reinstallation on more than one occasion to get back up to speed. Damien helped when he got around to it, but he was too busy to help promptly and (at least at the time) was a one man show.
I also find the multi-device experience and the discoverable links embedded throughout Roon to provide way more FUN! Cheers,
Spencer