#1
How would a maxed tube amp, lets say the Marshall DSL15 react to a clean boost (say the MXR Micro amp)? Would it have no effect due to the power amps already being maxed out or would it still boost the volume? 
#2
It'll boost the distortion. As you suspect, the amp can't produce any more volume, but pushing more signal into the amp will further overload it (mostly the preamp), resulting in more distortion. 
#3
Quote by Roc8995
It'll boost the distortion. As you suspect, the amp can't produce any more volume, but pushing more signal into the amp will further overload it (mostly the preamp), resulting in more distortion. 

Thankyou