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In other words the attenuator is slaughtering the tone of the amp?
Quite possibly, within 1db is more like attacking it with a wet lettuce.
I get your point though, and i am not saying there aren't a dozen things i am failng to account for in the tests and measuring.
If you look at a real plot of unnattenuated not zeroed it is just as all over the place, and the attenuated levels are pretty darn close. I just zeroed it in order to show deviations.