The best description I've ever found of what to listen for are those provided by Lloyd Walker for fine tuning your turntable. Since VTA and VTF interact, he recommends an iterative process of changing VTA, then VTF, and describes what to listen for:
www.walkeraudio.com/fine_tuning_your_turntable.htm
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www.walkeraudio.com/fine_tuning_your_turntable.htm
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