So I managed to find an alternate store that had one left. Feeling pretty lucky, as these are hard to come by these days.
Intriguing listen for sure. I would say I'm wow'd by them at first listen. Gonna hold off my complete impression until ample time and ear-adjusting to them. What shouldn't be a surprise is the bass is what grabbed my attention immediately, for better or worse. I knew these things were gonna be bass-strong, but it's more than would've thought Senn would do for a flapship. And I've only listened using my neutral Hugo 2 so far.
Unless you're a basshead, I would not recommend this is an only headphone. But in a collection, it's definitely one to have. To me it's a properly done IER-Z1R, which to me was a super fun IEM but missing refinement that the IE-900 corrects: a much less intrusive dip in the mids and crisper treble. It has a similarly wide soundstage like the Z1R, and maybe even stronger low-end (going by memory, as I haven't had the Z1R in a while). Senn also always has a natural and cohesive tone that I appreciate.
At this early point, I don't see this replacing my KSE1200, and honestly, it isn't technically close. It's easily discernable when analyzing the bass, since it's so loud. It's a good quality but doesn't have the micro details that the KSE and my Utopia reveal so easily. I am quite impressed with the IE-900's treble tho. It's super crisp and articulated without going over the edge to peaky. I am quite used to the KSE's treble though, which can be fatiguing. In comparison, the IE-900's treble is just right.
With fit, I totally get most peoples' criticism with seal. It was my hope that Senn would've learned from the IE-800's tips, but they're done in similar fashion here. They're stubby and and too soft (which affects both the chance of a good seal and isolation). Just like the IE-800, I have to shove them deep in my ears and play with them a bit to get a proper seal. But I guess nothing I'm not used to. Just something Senn should have improved by now. Fit can make/break an earphone; it's an important thing.
I like the look/feel of the cables a lot. I get peoples' complaint about the ear hooks/memory wire. My KSE has a similar offense. So again, if I wasn't used to it, I'd be complaining too. I have a Linum SuperBaX cable that I may try, if the MMCX socket fits.
All-in-all, I foresee the IE-900 being the set I have for causal listening (i.e. while working, travelling, just want to chill or want a bassy listen). It's the most balance I've heard between pleasantness and detail. But if you're an analytical listener like a lot of us on Head-Fi are, I would not recommend this as a primary headphone.