2008 Chevy Silverado Bose stereo system

I had what I tought was a reception problem, the dealer ship found nothing. The sound would go "flat" and then come back. Two month later they replaced the reciever but nothing really improved. Now 2 weeks later I am losing volume and bass randomly but for long periods of time. When I fisrt got the truck if I turned the volume to 3/4ths up I couldn't stand it. Now I can go full volume and still have a conversation with a pasenger in the front seat. What could be the root problem or multiple problems?

Alex