A competent piper (which I consider myself, just barely) should have the capability of adjusting the reeds within a range of volume, but the range is admittedly small.
I use synthetic drone reeds which are pretty amenable to tinkering, but haven't found anything other than cane reeds suitable for the chanter. With cane reeds most of us try to select one with the desired pressure and volume, because adjusting them physically does take some skill which only the most experienced pipers generally have.
Usually "lighter" reeds are quieter (relatively speaking) and the stiffer reeds are more stable. I generally keep a "good" (i.e. stiff) reed for outdoor playing (always used the good stiff one for competitions when I was doing that) and a soft, "quiet" one for indoor performances like churches and auditoriums.
Once the pipes are set up with a particular set of reeds though (and it takes a fair amount of time and tinkering to change the setup) there are only two volumes, "on" and "off." And the difference between a loud setup and a quiet one is really not very much.
-Glenn