From the way you described it, it seems your problem may still be in your receiver. Since your receiver had been sent in for repairs, that seems suspicious to me. If one of your Monitor 60s hooked up to the center is also making audible distortion, then to me it seems to pin the problem on the receiver. Now it doesn't sound like it could be your speakers, but what you could also have done was to connect your CS2 to either the left or right channel to further verify that it isn't the center.
Before you send the receiver back to whomever repaired it, try this first. If you know of a friend or buddy who has a receiver, ask him if you can borrow it. Then swap out your receiver with his. If you hear no distortion, then you know it's your receiver that's causing the problem. If you do hear distortion, then it's probably your speaker(s).
Give that a try. Good luck.
