Yeah you are right. It is not likely a fuse. I had a similar problem and it turned out to be the head unit. Before changing it out I'd try both front an back pre-outs on the head unit. Check the fader settings first. Maybe only one set is blown.
If you really want to save money you could try the high level outputs to see if they still work and then buy a low level converter (some amps have one built in). This solution obvoisly results in worse sound quality, it is just a matter of what you are looking for. And most likely if the low level outputs don't work, the high level ones won't either.