Jmark, thanks for the answer. To be honest I really wouldn't feel comfortable doing too much stuff inside the mixer myself but it's nice to know what to look for when I find someone to have a look at it.
Another strange thing i noticed, and I'm not shure if this has something to do with the previous problem.
Let's say I have Phono 1 turned to 4 or 5, and have the Master turned to 8 or 9, the bass response seems much better than if the situation is reversed, ie. Phono 1 to 8 or 9 and Master to 4 or 5.
Does anybody recognize this?