ADTech did some interesting tests that show how weak signals are damaged when a preamp is overloaded by strong signals:
Just the antenna, no preamp:
This is what the same signals look like when the preamp is moderately overloaded. Notice that the weakest signals are damaged first. The noise floor rises from the spurious signals created by the IMD. This reduces the SNR of the weak signals to less than the required 15 dB minimum:
This is what the same signals look like with a badly overloaded preamp:
This is what the same signals look like with a high quality preamp that isn't easily overloaded:
