You are probably burning your voicecoils of your speakers with an underpowered amp. This might sound strange but, because your amp hasn't got the power to drive your speakers it will put a DC voltage on the speakers. Because speakers have a very low DC impedance there will flow a higher than normal current through the voicecoils and they will heat up and eventually burn up. So my advice is: buy a more powerfull amp.
If an amp is overpowered and you are driving your speakers too hard you will hear the sound distorting beacause the speakers will reach the limit of their driving range. So in fact it is always better to have an amp that is a bit more powerfull than what the power your speakers are rated. It is also important to look at the impedance of your speakers and what an amp can handle. If speakers have a lower impedance then your amp has to work harder to get a decent sound level out of your speakers and eventually thrash your speakers.
I hope this will help you. This is a purely scientific approach of your problem so if anyone has a more audioholic explanation, lets here it.
