Whenever a question like this gets asked, the first thing that comes to mind for me is "Which do you mean? More gain or more volume?" I realize that the two are linked (more gain usually gets you more volume....usually), but the two terms are confused by folks about as much as tremolo and vibrato are confused.
So......do you want it to simply be capable of being louder or do you want it to be more intense in its distortion? Two somewhat different paths to follow.