The 'presence' control on your amp

[Matt]

Here's a handy article from Fender about the presence control. Many amps don't have this feature, especially smaller amps. Some harp players have this set really low because they don't want too much treble in their sound, but presence doesn't work quite the same way as the treble control and there is good reason to consider it when experimenting with your amp settings. While treble is not altered at different volume settings, presence is effected by the volume level. With a bit of trial and error you can add some 'bite' to an already dark harmonica tone without letting it get too bright.

http://www2.fender.com/experience/tech-talk/presence-control/