I'm poking a little.
But if I understand the video, that is the distortion measured at the speaker outputs.
That's 1% THD at the speaker outputs, that all the distortion combined from source to speaker outputs on the amps.
So if the line drivers aren't set properly you will hear it at about 1% distortion, if the EQ isn't set properly you will hear it at 1% distortion, and so on.
I don't want to make a DD-1 vs whatever other method of gain setting thread out of this.
Thanks Tony for this video.