I was always very happy with the `Fotofachlabor für Umkehrtechnik`in Munich.www.dia-spezialHis machine (sorry in german)
As far as I know the only lab developing Velvia neutral (without this magenta tones).
In his self developed machines (not hanging, don't know the english phrase but you can see pictures here
) and his technique he is even able to pull Velvia without color problems.
And yes: he does color corrections.
He knows every E6 film in the market and its characteristics. As I've heard he has his lab since the Olympic Games in Munich (1972).
Hope this helps