There's an interview with Fredrik
here
Among other things it says this:
Swedish Metal: That leads me into the next question. How did you guys came up with the idea to use the now well-known 8-stringed guitars? It feels like it isn't the most common step to take, when the bassplayer quit.
Fredrik Thordendahl: No, that's true. Actually, Mårten and I thought about quitting playing guitars. We though it would be cool to get ourselves two bases and play as hell with those instead! *laugh* But then we wouldn't be able to play any old songs, so then we thought of the idea to put an even thicker string to our guitars instead. One of our friends got us to meet a guy named Fredrik Thörnquist who constructs instruments at Newborn Guitars. He is fantastic at building guitars so he was very interested to do something for us. We thought that was a much better alternative than to play base. Then we could play all other songs too! But the funny thing with this story is that we haven't got those guitars yet! *laugh* So as of right now, there has never been an 8-stringed guitar in existance! *laugh*
Swedish Metal: It hasn't?
Fredrik Thordendahl: No! *laugh* And that is so fun since it has been so much excitement and rumors about those guitars that we were supposed to use on the new record and then they have never existed! *laugh* But we do have a prototype of it that I've tried. I've actually recorded some other music with that one, together with Jocke in Clawfinger. But on "Nothing", we play with our ordinary 7-stringed guitars that we've re-strung.
Kim