I just compared results between with/without attenuator.
I added a 20dB attenuator infront of BB60C.
I also added a 20dB offset in Spike software because of this attenuator.
Graph is almost the same, but now I have increased noise floor, so SNR is worse. From what you wrote I understand that this is sensitivity issue and an amplifier might fix this.
Also it seems that increasing the Max Input parameter increases the noise floor. I have to increase it because otherwise I get the IF overload warning. I hope amplifier wont make this worse.