Most likely the socket problem. Before I posted, I actually verified how it is supposed to work with a headphone jack and my 6500 - and managed to get the Beomaster into a state where the speakers stayed muted even with the plug out. I removed the plug, turned the Beomaster off & on and it worked again. A couple of times, when I removed the headphone jack, the relay click (and the sound through the speakers) only happened after a second or two. I guess this might be a common problem with the socket, oxidation or something.
Try inserting and removing the plug several times to see if you can get it to work again. If not, you will need to get it sorted from the inside.
Of course it might be a problem with the relay itself or the circuit controlling it, but the socket is my first guess. Can't be too expensive to fix - since you hear music through the headphones, the amp itself is still fine.