Previous research has implicated the existence of separate capacities for visuospatial and verbal working memory functions, observing modality-specific interference in dual-tasking paradigms. However, more recent studies have failed to find such a dissociation of memory functions, instead arguing for the existence of a single domain-general processing and memory system, with capacity constrained by the cognitive load of tasks. This study makes use of a relative-maximal digit span primary task, with visual and verbal secondary tasks, to explore the comparative impairments in true maximal-span recall given the apparent modality of the interference task. Verbal interference tasks are found to cause a greater impairment in maximal digit span recall than visual interference tasks, which are not responsible for any significant impairment compared to a no-task control condition. Potential confounding variables are addressed, and the results are discussed in relation to a multiple-component model of working memory, and with an attempt to reconcile contradictory evidence from competing theories.