not sure about what tone you're talking about...but a good place to start is to google search the artists rig, how they set up their effects, and then dial them in.  I don't have the 15, so I'm not too sure about how easy or hard that is to do exactly, but even with the bigger amps, it's hard to find some tones for some artists, you just have to make your own 'strange brew'

good luck