It is probably an interface problem with your cables (speaker and/or interconnects) This is the first I have ever heard of the Aragon rolling highs. I don't know of any amps of recent vintage(solid state) that don't extend well out past 20K.
Also, could it be that the Hafler was a touch bright and the Aragon is more on the neutral side?
Also, could it be that the Hafler was a touch bright and the Aragon is more on the neutral side?