I gigged the 20th Shiva for 3 years. Now I have an XTC 20th. If you have the clean channel volume up pretty high, say 1 or 2 oclock, the boost will add some nice plexi grit. I wouldnt call it crunch though. Otherwise, it mostly adds volume because the clean channel already has some pretty high headroom.
The boost button on the footswitch is for both channels.
Comparing the clean to the XTC...we all know the Shiva has one of the best clean channels out there, but Im in love with the cleans Im getting from My 20th XTC. Actually, its the best clean tone Ive had. Mostly because it has a full eq section. Having a mid control, really helps me to match it up with the drive channels.