I am finally on the DTS/Dolby Digital Bandwagon with a nice little Sony Amp and speaker set. One annoyance though - every time i turn the amp on for 40 mins or so I get a hissing noise out of the speakers, then it stop almost as though it finishes once it has warmed up. Anyone with an idea why this is the case, is it just a case of cheap speakers?? Sorry to sound 'fick' to all you experts out there, any advice would be gratefully received. Thank you.