You don't say which model of amplifier you are using. Speakers present a resistance to an amp output, usually only around 4 to 8 ohms. Also known as the impedence of the speaker. If you start adding more speakers to the same output terminals you actually alter the load on the amp output which can in the long term damage the the output stages of the amplifier. The volumes of sound I would imagine you're using would not be much, but I would consider the use of isolating transformers. Go into your nearest Maplins and tell them what you want to do. It would be worth it if it saves your AV setup. Good luck.