Sounds to me like you were expecting a more balanced colored sound from RE-400B. These are considered to be neutral, revealing, though more on a warm side without to much of in-your face high frequencies. I really enjoy their sound, and they are in a class by itself as my new "reference" headphones. I love my W40 and ATH-IM03, but I can't get the same level of details from those. If anything, I would compare RE400B to Ety's, but a bit warmer. As a matter of fact, I'm getting a regular RE-400 just so I can enjoy it with X5 and be able to compare to other IEMs in my collection. Because of RE-400B being balanced, even with adapter it will need quite a volume boost to listen from a regular DAP (assuming balanced headphones swing +V to -V so V is cut in half, instead of going full swing from V to the ground).
So, I can only suggest that either you don't have a good eartip fitment or just not used to neutral sound ;)