I have a miniDSP UMIK-1 that I use to set up the system when I change components. Was planning to use that. I did not know that SPL meters on phones were that accurate. That is easier than using a separate mic.
My WXC-50 outputs optical and coaxial simultaneously, so planning to feed the D70 with optical and D5 with coaxial. Then take RCA out from both and feed different amp inputs. This would help switch amp inputs on the fly. I will be using something at 44hz, so optical and coaxial should not make any difference. Still, around halfway through the test, I will switch coaxial to D70 and optical to D5 to see if it makes any difference, which I believe it should not.
My I suggest playing back 2 bar loops rather than full tracks, as this will allow you to asses any differences better, as as music is changing dynamically constantly, you could mistake DAC having more "punch" than the other simply as the musicians played a little harder at that part etc.