I am a impact/slam/bass head addict. 
I can’t directly compare the Volur to the IE900 but I can against the IE600 that is one of my favourite IEM’S for its slam/impact.
I would say if the IE600 is a 8 out of 10 for bass slam the Volur would be a slightly better 9 out of 10.
Must admit that extra point in the sense of punch/slam on the Volur may be due to the anti shout tuning of the Volur making me turn up volume slightly louder and impart giving the extra sense of bass slam/impact.
Bass quantity is pretty much the same BUT the quality of bass in the Volur is better. More detailed and textured.
I think were the Volur clearly wins out for me is in the feeling of separation and imaging and for me it does not require any EQ for me to enjoy. The boost in the air region spike does not bother me and I enjoy it.
The IE600 needs EQ for me, as the treble spike around 8k to 9k is a to much. I EQ this out and it sounds great. I think the IE 900 has a similar issue in the treble and also a big scoop/dip in the mids that the IE600 does not.
I Sometimes add a bit of a boost in the sub bass region on both iems as like I said this is what I enjoy the most. Mid bass amount in both is perfect for my taste.
If your searching for the best slam/impact in a IEM the IE600 is excellent at this and is waaaaayyyy cheaper than the Volur and I find them both super comfortable mainly due to size and weight there both very good in this regard my only complaint about the IE600 is the connectors as these can give issues. I am on my second pair as the first two were replace under warranty due to connectors not making good enough contact and I use them at home and very careful with them.
This issue seems to have plagued a few owners as per forums online.
Other than that can highly recommend either set.
Hope this helps.
