That seems like a rather flip comment.
Consider this. You've heard live unprocessed sound, right? So you think you know what a live piano sounds like? A few years ago I was shopping for a piano. Every piano I played sounded a little different, some dramatically so. And that is true of EVERY acoustical instrument. Every live venue sounds different - Chicago Symphony Hall does not sound like Boston Symphony Hall. Different locations in the same hall sound different. I'm very used to the sound of a live piano - I have an excellent 7' grand piano at home. But the sound of a piano at the player's bench is not the same as the sound of the piano for someone listening to it seated several feet away.
The first Stereophile test CD had J. Gordon Holt reading an article he had written, recorded using several different microphones. His voice sounded different on every microphone. So when you're listening to an "unprocessed" piano recording which you THINK sounds like a piano - what microphones were they using to record it? What hall was it recorded in? Where did thy place the microphones? What did that specific piano sound like? If you think about it, there is a processing step in even the most purist recording, and that is what microphones are chosen to do the recording, and where those microphones are placed. Because every microphone sounds different, and every location for those microphones sounds different. Those are the choices of the recording engineer. And by and large, we as consumers have no fucking idea about either, because that information is rarely published, and even if it were, most of us wouldn't know how to interpret it anyway. At best, the most we can say is, "gee, that sounds like my memory of what A piano sounds like." Note I say "A" piano, as most of us have never heard THE piano that was actually recorded.
A mastering engineer is at least several steps closer to the original sound than you or I will ever be, unless you record your own reference material. They at least might have heard the original sound, in the studio or hall, perhaps chosen the microphones, have some idea of what was laid down on the tape. And, Katz has said that although the majority of his masterings have required some "sweetening", some have not, so he has certainly heard and mastered unprocessed recordings. And even if he is using "processed" recordings, he at least should know better than most what the recording "should" sound like.
Look, I'm not saying that Katz is the be all and end all. I am saying that he is a very successful and experienced sound engineer and his descriptions and opinions are well worth listening to. Specifically I take his evaluation of tonal balance seriously because that is something that mastering engineers tend to be very particular about. But for the rest of us, who are using recordings where we don't know what microphones were used, where they were placed, what the original instruments really sounded like in the acoustic space they were recorded in, well... And that's assuming we are using acoustic instruments recorded in a live space for reference, and not a processed studio recording that never had an independent existence to begin with.
Sure, one can criticize what amp he has chosen to test the headphones with, etc. That's perfectly legit. Any subjective review is only a guide, anyway. My tastes and priorities may be different, my reproduction chain is almost certainly different, there is definitely room for honest disagreement. What a dull world it would be if everyone agreed with me!