I read everything the op wrote. The issue im reading he has. (I have an oscope and a DD1 BTW) When you are testing with a DD1 your getting a clean signal at 40hz. he wants to know how you know other frequencies are clean that you are not testing.
I have personally scoped output at 30hz on an amp after using a DD1 at 40hz and the signal look real nasty at 30 or sometimes 50. I think he is asking for an answer to that, rather then people telling him look for a video or explanation about it.
He doesnt have a DD! but an oscope and from his findings, he was clean at one frequency but dirty at another, he would like to know how the DD1 deals with these things that he is seeing on his scope.
I liked his long post as it was informative about the issue he had and was pretty concise about what he was looking for.