A little update: it could be your amp/headphones.
It's a pretty long holiday here in Thailand, so i did a little experiment and research.
1.) My M-100 are the worst offenders. I noticed things were a little more centered when un-amped with my rx-3b. So i tested them straight from my iphone. Definitely tilted to the right. I borrowed my wife's Beats Studio (i know, i know) and everything was waaay more centered. But seriously, it was hard to tell where things are with these cans. Took them to the shop from where i got them, the dude listened to them for a minute or so... And i'll hear from them on Wednesday whether i'll get a replacement or they can repair them!!!
2.) Now for the others (xba-40 and Woodies). I've noticed for a while that on the lowest volumes, my rx-3b is waaay louder on the right relative to the left. Didn't give it much thought - pots do that, i guess. But then i got to thinking that may be this is connected to things shifting to the right??? I read up on this a bit and concluded that there is a significant tolerance in two-channel potentiometers (even good ones) but maths says that it is much worse at low values and nearly negligible at higher values. So i tried setting my 3b on 'low gain' and turned the volume knob (nearly) all the way (needed to, low really means low with these guys). And yes, while the voice is a little to the right, it's a lot less now. Most actually seem centered as far as i can tell. And for some strange reason - and i might be imagining things here - this gets better (more centered) and the amp warms up.
Now i guess guys with sensitive iems or ciems have it worse - i barely noticed the imbalance with those beats as the volume is turned up quite 'far'. But I digress...my point is that if you experience this, it's best to check your amp/cans also. Specially the amp thing (theThirty3rd, that'd be you with them 334's - yes, i'm jealous but that's beside the point.)
I'll look further into this phenomenon/problem/time vampire and report more findings should i unearth any.