I have a different problem. I mess with many different things but still get the same sound always. Maybe you need to continue "career mode" and thing will be unlocked along the way. --- I don't have nothing interesting to say so I better not write anything at all.
To access Amp mode you need to score at least 70,000 points on any song. the easiest way to do this is to play the single note arrangement of Song 2 by Blur. It's not a gimme but it shouldn't be too difficult.
other good candidates are the combo arrangement and the single note arrangement for Go With the Flow by Queens of the Stoneage.