ST 70 screen grid and cathode resistor questions.

I have been planning to go ahead and change all of these resistors in my two amplifiers. My question comes from the fact that my buddy has told me that I could go all the way down to a 100 ohm screen resistor and it will help to prevent "ringing" in the output transformer. I do trust this guy's advice but 1000 ohms is what is on each diagram I look at and has been settled on and I just wondered if someone could explain to me why and what might happen if I went down to 100 ohms as my friend suggests.

Thanks!