Good day, thank you to everyone who took the time to share their knowledge.
I purchased an SMSL SU-1....then purchased another one....but that is not the whole story.
I hooked up the SMSL to the Airport Express optical output. Definitely a step up all round, I am useless at all the jargon...soundstage, dynamic range....safe to say, to my ears, it met my expectations and was much closer to the sound I was enjoying direct from CD.
I did trial Exact Audio copy and made some flacs. I followed an online guide to tweak the settings
EAC settings
The rips took much longer, to be expected with the more serious error checking, compared to iTunes. Some albums took 1 hour and it was not due to any obvious physical defects, I have always played a CD a few times and then stored it away, once ripped. It was interesting to get the EAC report, so far, clean bill of health on the rips made with EAC.
To my ears, the FLAC files played no different to the lossless ALACs I had created with iTunes, which is a relief as I do not want to re-rip everything. This was comparing the playback in Foobar2000 with the SMSL USB connected.
I connected a laptop to the SMSL via USB, with RCA out to the A85, there was no noticeable difference between using the SMSL fed from the optical of an Airport Express. This might be subjective but it does let me keep the option to stream to the Airports...but that will probably be never.....given:
Where things did get spicy were discovering 24bit / 96khz+ tracks. With FooBar2000 I played some well regarded HD tracks via USB to the SMSL.
If nothing else, the ability to play HD tracks has been a complete revelation.
As I mentioned right from the get go, I like to understand the tech and this exercise has been very worthwhile.
To celebrate I purchased a 2nd hand pair of Dali Mentor Menuet original editions.....awesome in every way.
I might have to trade my horse in for a normally aspirated diesel next.
Thank you again for the valuable input.