To be clear, technically, headroom is NOT a signal, or clean signal. Headroom is the "space" a signal can swing within the power rails of an amplifier without hitting the limits of the available power, both positive and negative. If you viewed the video one has the good picture of the "room" or space" between the average signal being amplified and the room or space being provided by the amplifier's "headroom. What the video doesn't show you is the negative swing of the signal. In any amplifier that you use, you have ground, positive rails and negative rails. The signal must have the same rail voltage on each side to amplify cleanly the signal to be produced. Music is an AC waveform. So it needs power availability each way. In an amplifier, if for some reason there was a problem with the negative voltage rail then you could clip the negative waveform before the positive waveform. This can happen for a lot of different reasons, like if you have bad solder joints on the big filter caps on the negative side (BTDT). That would be then "asymmetrical clipping" Obviously in the guitar world distortion takes on new meanings of bad distortion versus good distortion. In the p.a world ALL distortion is bad except for guys who want distorted vocals on purpose (punk stuff, or pain metal I dunno). So as you can imagine, in the p.a. world with massive swings and logarithmic amp power fluctuations, we hate distortion, and limiters (not compressors) are used (on amps and DSP feeding the amps). Headroom then, is not just one aspect of a circuit it's the WHOLE circuit. And it's one of the first things I have to teach noob sound guys. Headroom must be the same and as much as possible for all the circuits within the stages of a mixer for example. You need a good 20db of headroom at the preamp stage, the eq stage, the faders and summing, the subgroups, the master, etc. They should all be the same. If not then you could be clipping the crap out of the preamp (input to the console) and have 50 db of headroom at the outputs and that would be stupid. That is called "gain structure" and is the most important thing a sound guy needs to learn first when learning how to operate a mixing console. It's truly sad how many sound guys that think they're hot rod mixers don't even know what those numbers actually mean written on the board and what they are referenced to . Ask them what -10 means in the real world and no one ever really knows.