STR-DE697 reduced sound output from inputs other than tuner

[robwaysch]

I picked up a used STR-DE697 (less remote). I am attempting to use this receiver in a 5.1 configuration. I have HTPC supplying audio through an optical cable on the video 2 input. I also have an iphone hooked to the CD input on a headphone jack to RCA cable. Since I got it I noticed that the sound seemed muted. I have to turn the volume up to 40 to follow the TV/movie being played from the HTPC. The two scenarios I described above are the only ways I used the receiver so I initally just wrote it off as this is the way this receiver is (but in the back of mind I felt like something wasn't right). I decided to mess with it more today. I noticed that if I put it on tuner and find a station that I get exellent sound. The kind of sound I would expect to get. It gave room filling volume at just level 20. I tried moving my iphone RCA inputs from CD to DVD and also to AUX. All gave the same muffled reduced level sound. I also tried going through all the setup/level/eq settings to see if something made a difference. I tried different speaker configurations. I next tried clearing the receiver memory and starting from scratch. No luck. If I am any other input other than tuner I have to turn the volume up to 35 or 40 to get comparable sound (even then it is a bit muddled and not as bright as the tuner sounds). 

I have checked all connections, but connections must be good for me to get such nice sound through the tuner, right?

Just curious if anyone has any brilliant ideas or has encountered this problem. 

[robwaysch]

I should have mentioned that this reciever replaced a STR-DE485 receiver that gave 6 years of service with the same input sources before it began to pop and flash a "protect" message. I feel fairly confident that my sources are not the issue, but I could be wrong.