Receivers and Audio Components

HI all I hope some out here can help point me in the right direction I have skill but without the proper knowledge im screwed . I have a Sony STR-D965 surround sound reciever . I get great sound from all speakers  and proper delay on the rear but all I get is a muffled center channel that can hardly be heard  all the leads on the rear are good the speaker has been swapped and it works ok . put new wire on the center outlet same result  any ideas . I have it all apart and nothing looks burnt or smells funny .

I just got a Sony BDP-S550 Blu Ray Player recently and have it hooked to the receiver using the 5.1 RCA inputs.  Before I got my RCA cables I was using the optical input and the subwoofer was very loud (although you can't set your speaker volumes with the test tone as it doesn't work with the optical).  After I hooked up the cables to the 5.1 I did the test tone and you could just BARELY hear the sub, I had to turn up the volume of the sub to hear it on several movies and certain other movies I had to turn it down.  My question is...