Id use the time to prepare urself for the harder subjects, as some of them contain concepts that take time to get your head round. Getting your head round such concepts at an early stage will make your life a hell of a lot easier in Naples.
Have done nearlly all the ATPL subjects myself, and reckon it would be a good idea to pre study:
Principles of Flight, General navigation and Performance, as these subjects are more about understanding concepts rather than learning. Once you have understood them once, most of the hard work is done and the concepts will be ingrained in you brain for the future course.
It would be less useful I think to pre study things like Air Law and Operational Proceedures, as these subjects are based on pure learning of facts, which you will forget in five minutes and have to relearn fresh when you start the course.
Whilst the Aviator says that 6 mths is plenty of time to learn the subjects from fresh, I have felt very pushed for time during my ground school course. Anything that you can learn now and stamp onto your brain will make your life a lot easier later on.
Just my opinion, hope it helps.
Badboy.