Sorry for necro-ing my own post, but my 600s came back from Senn UK yesterday. £54 very well spent :)
New earpads and headband pad (the 650 one, interestingly). And two new drivers. The service note they sent out is a bit lacking in detail - all it says is "audio level problem - headphone unit - cut/broken - replacement", then there's the product code for two lots of what Google tells me is the driver unit.
End result, the soundstage is now rock-solid and dead centre.
A pretty fast turn around as well, considering I sent them just before the Christmas/New Year break.