question about fm radio sound

I have read the threads on the problem with the fm radio sound deadening off and on that myself and other people have experienced. My question is if it is a problem with the tuner then why does it not do it when I am piping my sirius signal through with the fm transmitter? I would think that it would do the same thing if the tuner had a problem. I wondered if the rds feature was causing the problem and not the tuner. It seems to me thats when it does it when the stations are rolling their call letters across the screen. Also is there any way to make the aux input any louder than it is? I have to turn my sound all the way up and can't hear it very well when windows are rolled down going down the highway. Are there any in line amplifiers available to fix this? What do you guys think? Thanks for any info. PS I have the 6 cd changer that came with the prefered package.