The stick is always the quietest stadium to watch on TV. I've never been, but most of the time I see them on TV the stick is like a funeral on third down and if the Cowboys, Packers or Eagles happen to be in town half the stadium is wearing their uniform.
This is the only issue I've had with loving this team (besides the losing, of course). From what I hear from you guys is the average Niners fan at the game just isn't willing to scream and yell and that's a shame because the average Eagles fan is.
I don't buy the sound amplification excuse either. Instead, I think us losing has helped our fans and helped us cherish winning more because when we are winning games (e.g the Washington game of last year) I DEFINITELY could hear a difference and it has nothing to do with the stadium.
If we start winning, have a great year and get in the playoffs then I expect us to have one of the rowdiest home crowds in the playoffs.