Well, it does prove that it is perfectly reproducing the music. Doesn't seem to me that you can go much further than that.
Yet, I'm going to presume you haven't listened to much gear, therefore, you don't trust your ears.
Double blind testing has demonstrated that ears aren't to be trusted. Seems to be true of noses as well: the case of wine pricing fooling the experts in to favouring cheaper wines thinking they had cost more.
The O2 is a great value ODAC as well, However I've heard plenty amps that are better, of course, IMO. But you'll just chalk it up to my love for distortion.
But then again, I've a harmonic distortion filter and have found that even the 'euphonic' even-order distortions are unpleasant the more you ramp it up. There's also phase issues that can give a 3D effect (ie. affect the perception of soundstage) though dsp is better for that purpose. And let's not forget output impedance issues. On top of all that are matters of taste. I prefer to be 'in the orchestra' rather than in the audience, so those 3D phase effects might make me think an amp is 'better'. But strictly it's not.
My own theory is that some people are also just more suggestible than others, as is true of hypnotism. An amp's price, weight and largeness of capacitors could be a good vector of suggestion. Mix that in to the heady mix above and you can imagine the problems with reviewing amps sonically.