It's not the amount of drive, it's the character of the drive. Most hi gain amps use one or 2 stages to saturate (distort). Once the stage crosses into saturation, it tends to get muddy sounding with more drive. Driving more stages by a smaller amount tends to make a more harmonically rich sound. So if you turn the drive down a bit and add an overdrive pedal at the front, it sounds quite different, even though the total amount of compression may be similar.
There seem to be a couple ways to do front-end drive. 1. run an OD with some gain at reasonable level so the drive happens in the pedal or 2. run an OD pedal with no drive but the level maxed so the drive happens in the first tube stage of the preamp.
I have tried both and prefer #1 just because I can also use the pedal with a clean preamp setting and it doesn't overwhelm me with the volume change when I switch on the overdrive.