You are wise beyond your years Grasshopper. You talk of practical things which one can measure, feel and see. Which of these sounds the best in your subjective opinion? I have a hunch they are all pretty adequate solutions subjectively, but which, if any, makes you want to move, or the hair on your neck stand up? Which do you really like however practical or impractical it might be?
They all sound very similar.
The A30 has a replaceable OP-AMP which I swapped for a LM4562 and compared against my second stock A30.
The A30 with the upgraded OP-AMP sounds the best to me, but it is marginal and comes out the most in music with Cymbals and Harps.
But honestly this is with analytical listening.
I can plug any of my headphones into any of the 3 amps and listen all day.
Just the A30 offers by far the most granular volume control which to me makes it the best offering.
I've also cycled the DACs and they all sound similar to any of the outputs (can't test DX7 with another DAC, it only outputs).
I tested the other amps with the Fulla 2 (DAC section), Modi multibit, SMSL M8, SMSL M8A, Topping D30, FiiO K1 (with windows volume adjusted).
Most of them sound similar in terms of listening to every day music but each has their own characteristics, especially with the selectable filters on the M8/M8A.
Otherwise volume was the biggest factor, which is based on the output Voltage of each DAC specifically.
My favorite ones by My ears were M8 and the D30. For all around listening specifically.
So all I can say is choose what works based on what headphones you have / plan to own. Buy what is most versatile for everyday listening (if you plan to listen every day that is).