The Harman research says to adjust bass to your liking. The bass shelf was originally there because that's what room acoustics do when you take flat speakers and put them in a normal room, which was the basis for the target. From that there was some adjustments based on different user groups for the amount of bass, and the current shelf is what 60% of listeners preferred, but the research also says that more "trained listeners" prefer less bass, and again you are supposed to adjust that to your preference.

People hyper focus on the bass, but the upper mids and treble is a big part of the Harman target. If you put the HD600 response on the Harman target and compensate it to the target, the HD600 looks very consistent with the target above the bass.