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Using a reaction model that incorporates pion bound state effects andcontinuum results for proton parton distributions and the pion distributionamplitude, $\varphi_\pi$, we deliver parameter-free predictions for the $\mu^+$angular distributions in $\pi N \to \mu^+ \mu^- X$ reactions on bothunpolarised and polarised targets. The analysis indicates that such angulardistributions are sensitive to the pointwise form of $\varphi_\pi$ and suggeststhat unpolarised targets are practically more favourable. The precision ofextant data is insufficient for use in charting $\varphi_\pi$; hence, practicaltests of this approach to charting $\varphi_\pi$ must await data with improvedprecision from new-generation experiments. The reaction model yields a nonzerosingle-spin azimuthal asymmetry, without reference to $T$-odd partondistribution functions (DFs). This may necessitate additional care whenattempting to extract such $T$-odd DFs from data.

Constraining the pion distribution amplitude using Drell-Yan reactions on a proton