Next best exponential DAC quality level?

I'd say you learned an important lesson about DACs, price doesn't = performance. You could spend 10x as much and still not hear differences unless the DAC has been purposely built to have a sound signature.

Every DAC I've listened to has a different sound signature.  They are subtle, but easy enough to detect, with the caveat being that the source files are of good quality and your system is up to the task of revealing the differences. 

Spending more doesn't always result in "better".  Typically for things to cost less there have to be either huge economies of scale (think McDonalds) or compromises made.  It's up to you as to what constitutes "good enough".

As to how much you'd have to spend to realize an "exponential" difference in quality", I don't think that's going to happen.

Putting performance tires on a Corolla does not make for a track demon. Now would you deride the performance tires as snake oil since it didn't magically transform your Corolla, which by the way is a great car for daily commuting, into a sports car? Same applies to audio equipment. Nothing lives in a vacuum. Everything is system/synergy dependent.

To answer a few questions:
Speakers: Kef LS50 w/ dual PSA subwoofer

Speaker Wire: Mogami DIY cables

Integrated: Hint6

AmazonHD: The source content is what they call "Ultra HD" (24/192) and I was able to verify that it was being passed at the highest bitrate.

I think I have a sufficiently capable system to discern any differences. Truth is, I did pickup subtle differences, but again the differences aren’t quite worth the cost of entry (IMO). I also fully expect that my ear just isn’t tuned properly to pickup the differences... Which might be a good thing :)

In most other upgrades: amp, speakers, speaker wire, TT cart, TT upgrades I’m able to pick out something ’better’ whether it’s more detail, lower noise floor,etc. With DACs it’s def. more subtle.

I guess I’m surprised at my observations -- and maybe lesson learned!

My next attempt will be with more capable DACs: Denafrips, Border Patrol, maybe a BiFrost2.

op i would agree that kef ls50’s driven by your hint should be sufficiently resolving to hear differences amongst dacs - do try hi-res streams from tidal or qobuz or downloaded ones - recording quality does make a huge difference in being able to hear into the music and differentiating gear

and i also agree you should go to the next tier of dacs - denafrips pontus, mhdt, soekris, schiit yggy or gumby and so on... enjoy