as jojo said, one great thing (tho sometimes annoying) about trackers is that there are so many different ways of doing one particular thing, its really just a matter of picking which one suits your situation best.
btw, in addition to the interpolate option, the amplify also works on samples with no volume set already. so, if you do an amplify 50% on a bunch of samples with nothing in the volume column, they will all be set to v32.
also, there's a problem with jojo's last method; omitting the instrument number with multi sample instrumens causes them to use the previous sample to play the new note... which results in very strange things.
load a multi sample instrument with a lot of samples, and do something like:
ModPlug Tracker IT
|C-401...
|........
|........
|........
|C-6.....
|........
|........
|........
|C-601...
and you'll see what i mean...