Poor audio quality

I've bought a Panasonic plasma recently, which is connected by scarts to a cable tv receiver and a dvd-recorder. I want to set up my mac mini with the tv and my hi-fi setup, which is basically a Nad amp and cd-player which are about 15 years old. I originally tried a set-up on my macbook, using hdmi and firewire cable and adaptor,to test itunes and the ipod touch remote feature, it all works fine except the volume is very low through the macook, and the same through my mac mini. I'm using the audio socket in the mac mini with standard 3.5mm audio cabling to the aux point in the amp. I've tried changing the volume to low, medium or high on the mac mini, but whatever i do causes low audio either in the hi-fi and the tv. I tried playing a dvd through the mac mini, low volume too. I chcked by doing the same through the dvd recorded and the sound was perfect. Everything is connected correctly and firmly, could it be just the quality of the cabling thats causing the problem? Could it be that my amp needs to have a digital audio point?