Basically, you can't ever increase the level of anything without there being a point at which it clips.
"gain" on a distortion pedal, will generally not effect the output level much. On most pedals you have an output level control, and this is what controls the output level. If you have a pedal in your effects loop, and it creates a massive gain boost (a boost in level, not a boost in distortion), it's after the pre-amp and before the power amp, and it could create distortion in the power amp, which could damage the amp or the cab (I think).