'Love the now-full-blown discussion that's literally stemmed from the use of one or two words; very
Head-Fi.
In all seriousness, though, I do mean that as a good thing. We're all ultra-nerds about this stuff and all this "fickleness" is simply a sign of passion. It's what makes the hobby and the community, really.
If I were to put my two cents into this, there is a difference between
diffusion and
reverb in my personal vocabulary. Reverb to me is echoes (or shadows) radiating out of a note, and the note itself is clear, solid and defined. Whereas, diffusion is when the note
isn't clear, solid or defined, so it comes off hazy and spread-out, and it ends up
blending with its own reverb. Take a light bulb, for example. If you looked at a light bulb with 20/20 vision, you could see that bright, crisp source of light, along with the softer rays emanating from it. Whereas, if you looked at it through smudgy glasses, there's that
halo effect that makes the initial source more difficult to make out.
I don't know whether or not the same rules apply to audio reproduction, but in audio recording that typically comes down to phase. So, for example, when you listen to a drum recording, and the snare and overhead mics aren't in phase, it'll noticeably hurt the punch and the imaging precision of those hits. The snare won't punch nearly as hard, and the hits will sound like they're coming from everywhere (or a certain radius, at least), rather than a crisp, focused point in the middle. Again, I don't know whether or not the same rules apply to audio reproduction in IEMs, but it's food for thought.