Even though you are hitting your receiver with 0 to 2 dB you may not be transmitting enough gain out of your transmitters. Check to see if there is a gain pot on it and turn them up. You may have to use optical attenuators after that to get proper levels at the receivers but it will get your signal out of the noise floor. And like Lazarus said....dirty fibers can cause it. Also the levels hitting your transmitters need to be checked. Noise in, noise out.
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