<<I have no expertise in the subject, but I do wonder how different PRIRs should really be from one person to the next. Isn't human hearing very similar or wouldn't everyone hear sounds like music, speech, etc very differently? That seems contrary to the evolutionary purpose of hearing (presumably to hear, localize, and interpert sounds of dangers).
I wonder if you could get something close to a universal PRIR, by averaging together a large number of PRIRs taken on the same system/room (i.e. AIX).>>
Well that's just it. Without getting inside someone's head, we have no way of knowing how music played over speakers in one room would sound to that person. It's certainly true that there is enough similarity that I can understand your speech and well as my wife could understand your speech. But she could also understand it if you had a head cold or were speaking from another room. But that doesn't mean it would sound the same. Similarly, I can listen to music and appreciate it coming out of a $100 speaker system. But that doesn't mean it sounds the same to me as it would from a $100,000 speaker system. We are made so that there is a huge range of variation in how a spoken or musical sample can be communicated. But that does not mean that the sound of that sample is the same across individuals. The differences are real and measurable (by the Realiser circuitry, for example in our case; but also they would be very apparent with accurate enough measuring equipment). Evolution is indeed at work; but it is precisely the fact that there can be a large range of variation between one and another person's hearing apparatus, to allow all of us to accurately locate the source of sounds and many other things we hear, DESPITE how different the raw sensory input might sound to specific individuals.
As others have said on this forum, this is really an empirical, experiential issue. You simply have to have the sound going into your own ears, with all the minute variations in how far apart they are, how they are oriented, the shape of the outer and inner ear, measured. Then, when music is processed using these measurements, the effect will be much closer to what the actual experience is like for you, than if you listened by way of processing done for another person. When you hear this, you will be amazed that actually, in fact, these small differences in shape, size, and distance, have a huge impact on the final experience.