  One of the goals of the COMPASS experiment is the precision study of
light-meson spectroscopy with data for various final states. With
$46\times10^6$ exclusive events, the process $\pi^- p\to\pi^-\pi^+\pi^- p$
constitutes the flagship channel. Based on this data set, the results of an
extensive partial-wave analysis (PWA), using a total of 88 partial waves in the
model, were published. Along with it, results of a first study of the so-called
freed-isobar method were shown. Here, the fixed amplitudes for appearing
$\pi^+\pi^-$ intermediate states used in the conventional analysis were
replaced by sets of piecewise constant functions to extract the amplitudes of
the $\pi^+\pi^-$ subsystems directly from the data. In this first study, this
was one for three $\pi$ partial waves with $J^{PC}=0^{++}$ intermediate
$\pi^+\pi^-$ states. The promising results inspired further extension of this
method, by also freeing intermediate $\pi^+\pi^-$ states with $J^{PC} = 1^{--}$
and $2^{++}$. With this extension of the set of freed waves, mathematical
ambiguities in the model arise due to the much higher flexibility of the model.
We will present first results of these extended studies on COMPASS data along
with methods to resolve the arising ambiguities.
