OK, I tried Atlantis and MS Word/Calibre. Both are more or less equal. There are small differences... but nothing really important. It seems both have a problem with bigger fonts (for example for a headline) and if there is a bunch of CR LF both tend to delete 'em. At the end the difference to direct converting with calibre is relatively small. But that might be a problem of the source rtf - it's kinda quick 'n dirty. I have to try it with a text I wrote myself. Maybe it works better with that.
@tyche: Thanks. I will try OO too. Maybe that works a bit better
guys, a lot of good suggestions. I'll tinker around a bit and report about the results