Best digital format (non streaming)

That is what I suspect. The difference was too obvious to be explained with a bad DAC in the CD player (vs. the Modi 3). I need to read the (exhaustive) manual of the receiver and its setting to ensure comparability. 

Likely, there are some (crazy expansive) specialty outlets that let you buy and download high res files? 

Yes, there are but prices and repertoire vary.  I am generally familiar with classical offerings.

And only storage media will be computer/harddrive/USB/DVD-R/SSD? 

That's my preference since it is easier to store, organize, access and play than physical discs.

Or can I buy Dark Side of the Moon or Brothersin Arms in high res on physical media?

You can.  I did.

Darn! The better my system gets, the more I find it difficult to listen to 'old favorites' that are just recorded horribly. And lesser and lesser recordings 'make it' into the 'play list'. Now starting to look into classical music hoping to find some 'worthy' material. And relegate the 'old favorites' to play over a bluetooth box and computer speakers :-) 

@kraftwerkturbo 

I listen exclusively to Classical and would be willing to help you, but aren't you putting the cart before the horse?  For me the music is the thing and gear is meant to reproduce it.  I wouldn't however, change my listening tastes just because a certain type of genre sounds better on my system.  However, we always need new Classical listeners

You are completely right (and wrong) by saying 'cart before the horse'. 40 years ago when I got into 'music', we already found ourselves 'putting the cart before the horse'. We (handful of friends) found ourselves with a decent stack of CLASSIC music (16-20 year olds !!!). Why? Because back then, rock/pop recordings were JUNK, and we soon found that using those to evaluate our systems was USELESS. And yes, back then and to large extend now again, I (engineer) like hardware part game (constantly evolving, comparing, improving) at LEAST as intriguing as the RESULT (listening to a song or a piece). 

I find this behavior not really that unusual; it is almost the standard for example in my other (performance cars) hobby, where everyone is constantly tinkering with the car to improve the performance, yet no one really spends a lot of time to HARVEST the improved performance (for example spending hours per week on the racetrack). 

But yes, there is NO question that I (and those like me) have the cart before the horse !!! 

With that said: can you recommend some really (technically) good 5.1 SACD classical recordings?