Trying to match the sound quality of circum-aural headphones, such as the HD600, with an IEM is fraught with uncertainty.
Since birth, we have all learned to adapt to the way our hearing is affected by the shape and size of the external (pinna) and internal (ear canal) parts of our ears. This "filtering" effect is part of the so-called head-related transfer function, or HRTF.
With circum-aural (and, to a lesser extent, supra-aural) headphones, the sound is produced outside the ear; so it is affected by the pinna and ear canal in much the same way as sounds heard without headphones. These types of headphones are usually designed with "diffuse field equalization", which aims for a flat frequency response inside the volume enclosed by the ear cups. The properties of this volume are affected by the size (but not so much by the shape) of the pinna and by the effectiveness of the seal against the head of the listener; but there is not too much variation in these factors from one person to the next. For this reason, most people hear a similar frequency balance from any given set of headphones of this type.
With an IEM, however, the the pinna is bypassed entirely, as is most of the ear canal: the sound is produced inside the ear. In order to produce a "natural" sound, the IEM must be equalized to simulate the effects of the missing ear parts: diffuse field equalization will not work. Because there is so much variation in individual HRTFs, the best the IEM designer can do is aim for some sort of "average" and hope for the best. A complicating factor is that many people have significant differences in the HRTFs for their left and right ears: these listeners will perceive a different frequency balance in each ear, which has a negative effect on their ability to recreate a convincing soundstage. For these reasons, there is considerable variation, from one listener to the next, in the sound of a particular IEM.
In the broadest terms, there will be some agreement on the characteristics of different IEMs. Most people, for instance, find that the Etymotics ER4 has lean bass and lively treble while the Shure E5c has weighty bass and rolled-off treble. Finding consensus on the finer details, let alone on a comparison with particular headphones, is another matter altogether.
