McIntosh MA7900 DA1 DAC vs. DA2 DAC vs. external

The ARC input is a really nice feature if you are integrating it with a TV, otherwise no, it’s not worth upgrading or using an external dac. They all sound pretty much the same.  For digital front end components just shop by feature.  The measurable differences between dacs are so far below audible levels, it would be like trying to listen to your watch tick at a rock concert. 

@perkadin great analogy.  Unfortunately, too many people on here are fuming at what you're saying, swearing they can hear worlds of difference in dacs.  I have failed to hear it myself and chalk most of it up to listening bias.  I strongly believe in listening bias.  Heck, my speakers sound different based on my mood often times.  I'll hear what I want to hear.
Glad to hear you say that.  I just have not personally tried enough dacs to be able to say definitively that one is better than another.
Speakers are another issue. 

Interestingly enough, I didn't hear a big difference at all in my McIntosh MA7900 integrated and my Anthem STR/PS Audio m700 systems.  Both were essentially equal.  Speakers were different though (Tekton Double Impacts and KEF Reference 5).

What I meant by that was I could tell a difference in the speakers, but not the systems. 

Oh, one used the Mcintosh's DA1 dac, the other used a Bryston BDA 1 dac.  I swapped the speakers back n forth..  In both cases, the speaker differences were the same, but no perceivable difference in the two systems.