I'm trying to educate myself concerning valves, but still need help with some questions. Such as, can I alter R13 from 33k to 47k (or even to 100k) without upsetting the gain or rolloff of this ECC82 input? My original K4000 circuit used 100k, but I would like to use this later version which does away with the input cap, while still keeping a high impedance for my passive preamps.