A very simple answer:
Ahh…the mysterious zero db on receivers nowadays. It’s amazing how something so small can affect so many…or something. Many “experts” have many different “explanations” as to how the 0 point is derived by different component manufacturers. They are, of course, all wrong. All their talk of “clipping” and “headroom” is just fancy-lad talk for, “They wave their magic voodoo sticks of voltage over the thing as they cast the remains of long dead woofer spiders of divination and whatever the gods of audio decide – so let it be written, so let it be done.” The truth of the matter is as difficult to explain as it is easy to calculate. I’ll spare you the complexities of the mathematical figures (mostly because every time I look at the equations I flash back to my advanced thermalductionivity class and start flopping around on the ground like a fish – it’s a defensive behavior (much like a possum) that has got me out of more than one final) and I’ll use an analogy.
The 0 point on the receiver is much like a spring flower…a man eating spring flower. Well, technically, a woman eating spring flower. Ok, forget the flower…there are no flowers. Ok, let’s start again. This mollusk walks up to this crab….oh, wait, that’s a different one. Let’s go with the carnivorous gender hating bloom. The 0 point is like that. Understand? No? Hmmm….Ok, everyone knows that 0 is not very loud right? Right. You know that. Your wife knows that. Your mother-in-law knows that. Receiver engineers understand that you want things to be LOUD. For most of us, the louder the better. What better way to justify playing something LOUD than being able to say, “But honey, it’s only at -20!” I mean come on! -20? How loud can that be? And if it sends your mother-in-law running from the room…so much the better. See, just like a woman devouring spring flower. Sheesh.
