If it's got two air valves, one will be positive air, and putting more air in there will make the forks firmer throughout the travel (Which is what you want, right?). If they're the same as my old forks (which were Marzocchi but not drop offs), then this'll be on the right leg, but you may have to experiment (or RTFM). What do you mean the rebound ain't great? Too fast, too slow, too variable? There should be an adjuster on them somewhere to control the rate of rebound.