When I Heard single sided Magnets i almost automaticly threw away the idea of ever buying them.
My first thought was "wait, so one side of the headphones is heavier than the other one? That cant be good, right?".
I think there are too many things that can go wrong with single side magnets (and im not really skilled at electronics, just seems logical)...
Also I do have to note that even if they prove to be better sounding the price did go up, so... Its not that cool.
Im looking forward to hear opinions on the headphones when they actually come out anyway, since i Really enjoy my HE-400.
On a side note, wasent HE-400 relesed like in 2012? Feels a bit too fast to relese a line 2 years after the previous one, some people might feel like they bought something that was more like a testing product (like all the HE-400 revisions). I feel slightly that way but it dosent bother me that much since the price went up for the 400i... Just feel a bit odd (especially since i bought the HE-400 about a year ago).
As has been mentioned single ended has it benefits as well. Weight being one and also you dont have a magnet array blocking one surface of the driver.
Also I think 2 years is a good rate for headphone revisions, especially when correcting serious ergonomic deficiencies.