The TH900 Mk2 is an energetic headphone with pronounced bass and treble regions; it is "v-shaped" with distinct emphasis on a quality low-end region, especially in its sub-bass. Thanks to its well-defined highs, it can also be somewhat peaky and bright; thus, it won't fit your need for a darker, smoother sound.
The Eikon doesn't have the bass impact of the Fostex, but it has a sculpted bass presentation that provides sufficient impact, without being overbearing. Ultimately, I'd classify it as the most neutral of the closed-back headphones in ZMF's catalog, with a judicious slant towards the higher registers.
I haven't heard the AEON 2 Noire, but I have the AEON 2 Closed. The latter has decent bass - it has sufficient impact, with excellent sub-bass extension, and layering. It is also the
"darkest" of the 3 headphones in this comparison, with a slightly attenuated treble vis-à-vis the Eikon and the TH900 Mk2. Make no mistake, though, the highs have good definition, but it isn't strident, prominent, or harsh.
If you want a dynamic driver counterpart to the AEON 2 Closed, but with a heavier mid-bass punch, try the ZMF Atticus. They come with wood cups, too.