I've done the same while staging some 6500's in the lab, and had to use 10dBm attenuators to avoid overloading the receiver. Depends on how good your cabling and connectors are.
Requires 5 dB 1550 nm fixed loss attenuator for < 20 kms. Attenuator is available as a spare. The part number is WS-X6K-5DB-ATT=.