I have experience with the B2, the RE 400 and the R50 - I kept the RE 400 over the other 2.
I listen to rock, blues, jazz, classical & ambient.
What part of the music, if any, do you like to be emphasized or de-emphasized? That might help you reach a surer conclusion.
I feel the the B2 leaves out too much in the bass end and can be overly bright.
For me the R50 has great bass representation, okay mids and sometimes irritating highs
I really liked the R50s, they would work well for Radiohead yet when I heard the 400s I smiled.
The 400s sounded more like my big speakers than any of those others.
I feel they leave the original signal the most intact without adding or taking away
The vocals and guitars are never behind the rest of the music- at least in line or slightly in front
I use the Fiio E07K and on some tracks I'll put the bass up one notch - on some others I may put the treble up a notch - mostly things stay at zero.
I would not make a buying choice based on the accessary pack because you can't listen through accessaries.
Sound signature and comfort then accessaries.
The 400s came with a case and a bunch of tips (including comply 400s) - I went with the Complys - comfy fit, good bass response and easy highs. Zero listener fatigue even after hours of use.