If you think Garage Band is good, you need to take a serious look at Ableton Live. Basically, everything that Garage Band does, plus a boatload more, AND the ability to trigger all the loops and clips by MIDI or USB controller surface.
No.... not a chord following arranger, but a way to interactively trigger all the loops live, in realtime.
An amazing program.
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An arranger is just a tool. What matters is what you build with it..!