My experience with RE-400 is too short to say, however with RE-272 and RE-ZERO latter series it seems there's nothing you can do. The cable itself does not crack, but it presupposes the cable to break at the end of the strain relief. This had happened with my first pair of RE-272. RE-ZERO look "bent" at this point too. Not to mention the resulting springiness makes the cable hard to coil.
More on topic: Ok, these have bit more low bass than RE-ZERO and actually hit exactly as hard. Leckerton really exposed the bass potential of these while having more refined treble and causing these to control their highs reverberation a bit or smooth out some highest end peak or improve the extension. Not that they sound bad without amping.
Audiotrak Prodigy Cube does not have the same effect on highs, but has something similar with bass. The highs are definitely more refined with the higher end amp.
The only thing these lack is the amount of air that was in RE0. Perhaps a trivial tweak such as removal of the paper filter will fix it. If RE-600 will be any better at this... well...