If you are going to base audibility on measurements and not actual hearing tests, then you should do as much research into the limits of human hearing as you do into electronic engineering. Most audiophiles think their ears hear an order of magnitude better than they actually do. They invest bias into their hearing being superior to other humans, The truth is that human hearing at its absolute best performance is remarkably consistent, and quite humble. The variations in hearing are all related to degradation below that level of peak performance, not being able to hear better than the norm. But the simplest way to know if DACs sound different or alike is to just grab a range of different kinds and price ranges and do some controlled listening tests. When you take the time to do that, then you'll know. Audibility is where the rubber hits the road. When you sit down to play some music, it's all that really matters.