Recently I have been revisiting the top IEM from the list and comparing my impression with nic's review. I have become more interested in the particular aspect of 3-D imaging, depth and width of the stage. I personally think this aspect for IEMs is relatively vague or hard to depict / pickup compared to full-sized headphones that I previously / currently own such as HD800 and HE1000. At least to my ears. YMMV
Apart from the difference in the ability of imaging for the IEM itself, I realized the other important issue is that not every song in my collection is produced/ mixed to create a 3D image. Such as music that utilizes a mixer to produce / electronic music / synth pop / jpop / anison. Instruments are simply behind the vocal. Even if I revisit the tracks that I used for auditioning on full-sized headphones, some of the tracks does not create a 3D sound of a different instrument. The entire music sounded 2D/wide, and not 3D or with much depth, I think it is how to track is intended/produced.
IMO, while comparing the ability of 3D imaging, depth/height of soundstage in IEMs, the choice of tracks is equally as important to the raw power and capability of the IEM itself. I think most of the tracks I used when I audition VE8 or while making a decision between A18 / Zeus are produced in a 2D fashion and not intended for the listener to pinpoint the position of each instrument, I am not able to compare the aspect of the depth of soundstage between the A18 / Zeus. Going back to the VE8 / Zeus / Arthur with better-produced tracks has impressed me on their ability of producing 3D stage. The U18 (my A18 unit has not arrived yet) obviously sounded more flat, but has a much wider soundstage.
Although my choice between A18 / Zeus will still be the same even if I factor in the ability of 3D imaging and depth of soundstage between, it has been a great lesson for me and has broadened my experience.