Finally decided and went for the IE900 (was debating between that and the IE600). I didn't have a chance to compare the two, so what made me decide was the promise for more soundstage, resolution and deeper bass.
First impressions. Stock tips are not great. Normally I use M size tips, but only the stock S will give a proper seal. However, they don't make the IE900 easy to fit in the ears and require some fiddling. However, a pair of SpinFits CP100+ M has fixed the problem, though I'd still like to try the IE Pro tips suggested earlier in this thread in order to get to be able to use the foam filter (the SpinFits inner hole diameter is smaller than stock and the foam from the stock tips would compress inside, potentially messing too much with the sound).
I've tried IE900 with a Qudelix 5K, Violectric Chronos and directly connected to MacBook Pro 14. All will give me a buzz noise, possibly because of some kind of intereference. I haven't tried the balanced 2.5mm cable with the Qudelix to check if that eliminates the interference. All three sources sound similar to me without EQ applied. However, I will definitely not use the MacBooks internal DAC because movement on the screen or trackpad will trigger a ridiculous amount of interference (I can "hear" scrolling, for example).
As for how does the IE900 sounds? Bombastic. The bass is indeed awesome. I've compared it so far with the Airpods Pro and the difference is night and day. The probably fairer comparison shall be done later with my LCD-i3. This EP from Lord of the Isles is for me a perfect illustration of the IE900's capabilities across the FR spectrum: