Recordings that do not have pleasant-sounding highs produce treble that is not pretty. But change to a different recording with pleasant treble, perhaps an RCA Living Stereo, for example, and the highs sound like they are coming right off the tube microphones and mixing board. The extension is there without the top end being pushed forward or being aggressive in any way, unless the recording is this way. I heard more variation in the character of the sound of the top end from these amps than I recall from listening to any other amplifiers. The importance of the statements about these amps sounding like the recording and not having a consistent sound of their own should not be underestimated.
Most amps apply a sonic signature that is consistent.
Amps that produce the same effect consistently are inserting themselves into the music. In my experience, the Warner Imaging ER-300MSE mono amps set the standard for sound that replicates the music without altering it with a consistent sonic signature. The midrange of the Warner Imaging amps is as neutral as I've heard. Really there isn't much to report on in this area other than to say that the ER-300MSE amplifiers did their job admirably. An a cappella recording allows you to simply hear the human voice without accompaniment, which makes a reviewer's job easy when assessing this important area.