They use parameters or settings that don't exist on the actual amp. I was trying out a Helix today and pulled up a deluxe reverb amp model. Everyone knows that these amps get their grit from turning up the volume, yet the Helix parameters, and other modelers I have seen have a "drive" setting. So you end up with channel volume, plus a drive, plus an overall master volume knob on the helix unit itself. What is the purpose of the drive setting in amps that don't have one physically? It makes it confusing as to how to set up the amp because at that point it's not a real representation. Why not have it authentic and have the amp break up only if the amp volume is in the upper ranges? Ideally you would have the parameters change for each amp to represent what is on the actual amp if you were to be looking at it in person.