Michael, that’s probably because you are mostly using TOTL DAPs.
In terms of driving IEMs, the improvement from adding C9 is definitely smaller if you pair it with something like P6 Pro, compared to when you pair it with mid-tier DAPs like LP6K or R6.
Basically, if we agree that there is a difference between mid-tier DAPs and TOTL DAPs in their ability in driving IEMs, then C9 shall be a (potentially) useful improvement to the audio chain.
Edit: To quickly add, C9 is definitely doing something very noticeable to my N6ii (E02). My reference point is Hugo 2. N6ii is nowhere near Hugo 2 without C9. With C9, it becomes at least a draw to me with my various IEMs. Of course, we can talk about whether this is due to synergy or other things, but I just want to point out that, to me, the improvement it has for mid-tier DAPs is genuinely there.