I've had to make up adjustable line level converters in the past to run speakers off a decent amp that is fed by the speaker output of a B&O multi room system. Can't remember the B&O model at this point. I remember the adjustability being important though to get the output vs distortion ratio right. I had tried a couple of resistors but eventually found it much easier to just put in a pot and have a play with it.
I guarantee I'll tell you the truth and I guarantee I'll tell you what you need to know but I can't guarantee that I'll be telling you anything you want to hear.