Keep in mind that even if a booster is a "clean" boost, meaning that it does not clip the guitar's signal within the circuit, the amount of boost will determine if you get any distortion or not. This is because that boosted signal is now hitting the front of the amp. If that amount of boost is greater than the available clean headroom in the preamp, guess what, you'll get clipping.
So there is a fine line that you have to watch. The more clean headroom your amp has, the better off you'll be, if totally clean is what you're looking for.