The difficulties of evaluating DMIs (digital musical instruments), particularly those used by ensembles of musicians, are well-documented. We propose a methodology of rehearsal-as-research to study free-improvisation by ensembles of DMI performers. Sessions are structured to mirror established practices for training in free-improvisation and to allow controlled, order-balanced studies with extensive data collection in the style of factorial HCI experiments. Experiment structures, designs for questionnaires, and objective measures such as session duration will be discussed. We ask whether improvised rehearsal processes from music could apply more broadly to studying collaborative interaction.