I’m getting my gain tones from the amp, so if I hit the front end with a boost it will increase the saturation but doesn’t increase the volume so much. I put the clean boost in the fx loop so it comes after the amps preamp and gives a nice volume increase for solos without affecting the tone. The pickup booster is perfect for this as it’s a clean level boost.
If I was playing into a clean amp and using pedals for gain then yeah I’d put the pickup booster in front of the amp and after the gain pedals to get that increase in volume for solos.
For boosting the front end of a crunchy/overdriven amp into more saturation I use either a boss sd1 or Maxon808 in front of the amp, but still have the clean boost in the loop to increase volume for solos. The SD1 and Maxon808 also do a great job of tightening up and eqing the tone as they boost mids and cut bass a little.