I'm trying to encode cuts from a DVD source using avidemux, since they have to manually be resized from 720x480 to 854x480, but the major issue I have is that the Adaptive Quantisation (AQ Mode) needs to be set to 3, or the darker parts of the cuts end up wrong, but avidemux won't allow me to go above 1.50. Any advice on this?

1.50 is the strength, which you should probably leave at the default of 1.0. For AQ Mode 3, you just need to set the mode to "Auto Variance Biased".