@Essentials hits it on the head on a number of points. I find the IE 900 to have more bass than expected, on par or even more than the Volur because of how mild its upper mids are so you turn up the volume a little more. He’s right that the IE 600’s bass is not as good quality as the Volur - there’s a bit less depth and it’s more smoothed over. And if you have issues with the treble, both the IE 600 and IE 900 won’t do it for you.
But if we’re just talking about pure bass quality, I have to go with the IE 900. Something about it just scratches almost every itch I have. The word “impact” means something a little different for each person but IMO the IE 900’s bass is more raw than the Volur’s. The only complaint I have is that its leading edge isn’t quite as sharp or explosive as I’d like. But it’s subbass heft and sense of decay is second to none.
You can read my IE 900 review here cause I go into it with a lot of detail. Including some of the issues with these IEMs that @Essentials pointed out like its ergonomics… Sennheiser IE 900 Review: This is the Bass I've Been Looking For! – Headphones.com