Thanx for replying!
I also thought about Morrisound and Studio Fredman...
but I would add The Twilight Hall Studios (because it's Blind Guardian's studio...)
The producers I had in mind where Fredrick Nordstrom, Jim Morris, Charlie Bauerfeind, Flemming Rasmussen and Steven Wilson (if it was he that produced Opeth's BlackWater Park...)
oh, and The Gathering had a great sound quality on their last albums, so I guess they had a good producer and studio (don't know the names)...