Hi Alessandro,
Thank you for trying to help me. If you simulate an OFDM system from optisystem sample files (or probably if you simulate ANY system), the same thing would happen. The amount of amplification is the same as the amount of accumulated attenuation in each span. However, I believe the behavior can be well explained by noticing the fact that the Optical amplifier has a noise figure of 4dB. So when you omit it, the signal becomes less noisy but weaker.