Initial impressions, and may change, but this is my thoughts so far.
W900 sounds like iem verson of my HE1000. I guess it's due to the dynamic driver bass, which reacts like subs. You can hear the sub-bass separated or layered at times, and not all that forward in terms of bass. Compared to my Andromeda, not really all that forward. Also the speed of the sub bass being slower than Andromeda is felt. Due to it's sub bass, bass can feel very soft than punchy. Sometimes it sounds like subs to mid bass are a bit separated that transition isn't quite as linear as I've heard(probably due to separe dynamic drivers).
Some tracks W900 sounds clear than Andromeda due to I believe due to mids not being on the level of Andromeda, causing clarity to the lower treble, and certain sounds in the lower treble to come out better articulated(or details as you will call it), but also something about Andromeda sounds right in it's transient response to tighten the sound more.
So, the spearation of bass to rest of the frequencies are there that there is no bleeding of any sorts.
It's airy sounding and clear. The mids are more refined on the W900 over the 500. W900 has pretty laidback mids like it or not. I think mids articulation could be better. Perhaps it's not mid lifted as much as others, but mids articulation is an area I particularly pay attention to.
W900 is not dark, or really bright either. Treble level is just about right IMO. Vocals come off on the smooth end. I think this is the iem for those looking for smooth and perhaps Westone likers. I would guess Andromeda treble and bass will come off forward for most in comparison.
It comes off airey, and large sound stage, but not the largest I've heard(Andromeda's is quite large). Opus #2 really widens the soundstage.
Perhaps more to come.