There is an effect if the pulser is at higher frequency in the low energy part of the anodes (apart from the funny situation that the scaler shows an increase from 1.7 Khz to 8.5 kHz and the dynodes rate remain with the same rate)

The effect can be seen in the spectrum of the anodes. See screenshots in the attachment. First attachment (pulser at 10 Hz, second attachment pulser at 100 Hz)