Your "other IEMs" include the PFEs and DBA-02 (according to your inventory). Are you sure these seem muffled compared to the Atrios?
On another note, I had the v1 Atrios, their soundstage was pretty small and nowhere near the Sony's. I understand Future Sonics improved the highs on the newer version, but soundstage would presumably be still the same?
I was thinking about this and the Atrios clearly had a larger soundstage than the Miles Davis and a bit larger or equal to the mtpc. However, I had to insert them quite deeply to get a good fit and I think that's where a bit of occasional constriction came in.