@MachineGunz I think I understand what you are trying to say.
I owned (and used heavily) AD900 for a long time, and absolutely adored them (as I understand, AD900x are similar). You will NOT get 400i to sound like an improved version of AD900, they are very different phones.
AD900 had very wide soundstage, abnormally so. It was artificially wide. In some cases, it worked out (listening to symphonies), in some, not so much.
They also had some inherent strong grain, were light on bass (good quality, but light), and heavy on treble. I enjoyed them very much, but eventually moved on because grainy treble was giving me headache and achy ears after long sessions.
400i are known for their fairly 'closed' and 'narrow' soundstage - they are almost like closed headphones in that respect (hell, my closed T70 have better soundstage than 400i).
What you gain in exchange (with good amplification) is very smooth 'creamy' sound that is still fun and aggresive - there is not much grain to speak of. Also, bass is very nice, and treble is much better balanced.
Plus amount of details is higher. Also, bass and midrange have that characteristic 'planar' euphoric sound, most people like it. Ah, and they are amazing for long sessions - very low fatigue.
Again, 400i is not in same family of sound as AD900. If you are looking for improved version of AD900 (I was looking for that for a long time), DT880 or T70 are more like it.
Sorry if this is bad news for you, just want you to have realistic expectations. 400i are very good headphones, but they do not sound like AD900 at all.
P.S. I heard M50x briefly on friend's rig. Overpowering but flabby bass did not do it for me (but admittedly I am very sensitive to overdone bass, more than most). 400i are much more tasteful and detailed down there when amplified well - with H10, for example.
Ah, also, find out if Bursons lifetime warranty is transferable. If it is not, I would think twice given their failure rate in H10.