Well I'm sure I am not the only one mislead by the title of the post- the base is the part the speaker stands on! 😉 However it seems you are talking about adjusting the low notes in audio which are referred to as 'bass' instead- so I would say that you will not find any way to adjust it many apps other than iTunes. In iTunes, press command+option+2 (or choose Equalizer from the 'Window' menu) and you can use the Equalizer to adjust the sound to your liking- or use some of the preset curves to see if any of them sound good to your ears.