Hello all and thank you again for active participation:
This Friday evening I came in much better prepared: four songs clipped @ 30 second samples, same source (iPhone 13Pro using VLC), two systems to compare (Chord Dave vs Apple Lightning Dongle), volume matched using a small portable scope + double check using ears, and a simplified test: The test subject does not know which system is which, though he can switch between source No.1 and Source No.2. Sources play the same track, almost in synch. Subject can switch between the two sources as many times and as quickly as he wants and only needs to identify which system is which. My friend has scored 3/4 (got overly confident), I have managed to nail 4/4 tracks. So there is a difference! Have latter spend around two hours listening to the music and got very confident nailing down each DAC signature sounds. Tried the same tests using Chord Dave without the upscaller with similar results. Speaking of marginal returns, the both devices produced music quite well.
I would not be confident that I would distinguish then in a double blind ABX testing, but I shall definitely try in the future, starting with sequence A, B, X repeated by A, B, X. I suspect my confidence would go down if the listening sessions would be longer, lets say 1-2 minute of each system instead of available instant switch.
Testing is very much like work rather than enjoying the music, so I am sorry I did not perform 100+ hours of analysis, excel sheets filled with data etc. for you.
Speaking of.. sound level matching. Proven crucial. I used a portable scope, yet easiest was to set it using sine-wave, not pink-white noise or music. Using noise files the voltages were constantly drifting, I know the USB scope is more of a toy and not a piece of art, yet it is way more sensitive to voltage than my Voltmeter. Can you share your experiences or drop a link to a good resource on level matching?
Moreover, one more question on Tube amps. I have a Bottlehead Crack OTL tube amp, which I level matched to A90 headphone amp, using a scope + 1 kHz sine wave. Crack OTL sounded very much more quiet, then I volume matched using my ears and got 115 mV Vpp on the A90 and 155 mV Vpp on the Bottlehead crack. The Tube amp voltage also seemed to wobble a lot from its main axis. Anybody could comment on that? Screenshots attached.