Thanks for your answer!
That's pretty much what I was expecting. Finnish TV channels use subtitles that are mostly burned into the picture stream, because DVB subtitling is working badly or not working at all in most set-top boxes. Unfortunately Dreambox is doing as badly as other boxes in this respect. I can't use zoom/crop modes because the subtitles would be cropped.
I'll just have to keep using SCART output and wait for DM8000 and its HDMI or DVI output.