Thanks. Well, I do have a set of 4.A (not Ai, though I've heard the 4.Ai quite a few times), so I can try to compare the two, though it'll be my no means precise. If I compare between the 4.A and the Pro100, I'll notice that the Pro100 sounds noticeably cleaner, because the lower midrange of the 4.A/Ai is elevated. This elevation causes voices to be pushed much closer to the ear than compared to the Pro100. At the same time, the 4.A sounds more detailed and has more of an "enveloping" feeling that allows the listener to feel more engrossed by the music.
It's my understanding that the 4A and 4Ai share the same drivers, so they should have at least similar sound qualities. Interesting that you find voices to be more forward, and yet the sound is "more enveloping". This to me seems to be a contradiction. A top to bottom coherency of the stereo image gets me engaged. Accentuation of any particular frequency I might find a distraction. With the 4Ai I have similar impressions to yours, and I think the 4Ai is brilliant. "Engrossing" is a word you used. I feel the same about the 4Ai. I am perplexed at this but happily so. This leaves my with a lot to think about, and more questions to answer before I make a choice of CIEM.
I appreciate your musician's perspective. Musicians, recording engineers, and audiophiles all look for different qualities in their transducers. It really helps to know where you are coming from. It seems that you have an appreciation for what others might want too.