I may have missed a post about this. since I cannot find an answer, I will ask you if it is possible to use music software alike garage band or else on my powerbook with the TWA.
And if so, what connecting cable should I use for that?
You can certainly plug TWA into an interface to use in DAW software. I do know MIDI Guitar 2 and SampleTank are available for computers and not just as an app, so you may be able to use the backing tracks the same way as the iDevice adapter cable allows you to. https://support.tonewoodamp.com/support/solutions/articles/17000024243-using-with-idevice-video-