thats absolutely trash guide, when it buzzes like that its already hard clipping with a SQUARE wave and with mids and highs its already the point of no return, any longer than 1 or 2 secs is more than enough to pop a speaker especially above the 100 rms range of amplifier power. Soft clipping and distortion happens way before that and you are using a 0 db test tone which is extremely harsh because that puts your pre-out voltage signal sky high aka the point where you thought your head unit clips lets say for example 62 out of 62 is going to be far lower now around 56 ish due to the heavy signal strength. with mids and highs using music you can easily tell when the speaker is stressed FAR before soft clipping occurs because ears are very sensitive to harsh frequencies which are more inherent as you get higher on THD (still below 1%)