| Also, every active (powered) circuit will alter the signal slightly. No matter how many times a seller or reviewer uses the word "transparent", a booster is never ever 100% transparent. So then the question becomes "how much is the tone changed, and in what way?" If a booster is advertised as "really clean" or "transparent", then we're typically talking subtle differences... a certain je'ne sais quois... Words like "punch", "fullness", "fatness", "clarity" etc. are very subjective, they may mean slightly different things to different people. "Fatness" almost always involves some subtle/soft clipping, so if a booster is claimed to be "fat" then guaranteed it has some coloration or compression even if they are also saying it is "transparent".
Personally I like the sound quality and relative transparency of a class A boost, but even then different class A circuits may sound different due to component choice, voltage or current usage, etc. Some clean boost pedals advertise higher headroom if you power them with 18V instead of 9V. Higher headroom means less coloration, less compression (given the same input).
For general "rock" purposes, the difference between clean boosts live on stage just boils down to "clean" and "dirty". All the other subtleties are for quiet time at home, or in a delicate/sparse mix. | 
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