Like everyone said, you probably need to provide more info. I've seen people paint their wall outlets and the paint gets on the power cord or in the outlet and acts like a dielectric, but it sounds like you know better. You could have plugged a line level source into the phono input on your preamp, but I doubt that you would make that mistake, as well. If you really can't figure it out, I would take each piece individually, and try them in a system that is working.
I just thought of one other thing. You said you swapped IC's. Did you check the actual RCA connectors on your equipment. Sometimes the RCA connectors they put on cables fit very tight and can break whatever you attach them to. I've done that myself a couple of times. You have to check carefully, though. The connectors usually don't just break off. Most of the time the connection will crack at the soldier connection on the inside of the component. And if you have a partial connection, that could explain the muffled sound you are getting.
I just thought of one other thing. You said you swapped IC's. Did you check the actual RCA connectors on your equipment. Sometimes the RCA connectors they put on cables fit very tight and can break whatever you attach them to. I've done that myself a couple of times. You have to check carefully, though. The connectors usually don't just break off. Most of the time the connection will crack at the soldier connection on the inside of the component. And if you have a partial connection, that could explain the muffled sound you are getting.