Despite increasingly accessible and user-friendly multi-channel compositional tools, many composers still choose stereo formats for their work, where the compositional process is allied to diffusion performance over a `classical' loudspeaker orchestra. Although such orchestras remain common within UK institutions as well as in France, they are in decline in the rest of the world. In contrast, permanent, high-density loudspeaker arrays are on the rise, as is the practical application of 3-D audio technologies. Looking to the future, we need to reconcile the performance of historical and new stereo works, side-by-side native 3-D compositions. In anticipation of this growing need, we have designed and tested a prototype `Virtualmonium'. The Virtualmonium is an instrument for classical diffusion performance over an acousmonium emulated in higher-order Ambisonics. It allows composers to custom-design loudspeaker orchestra emulations for the performance of their works, rehearse and refine performances off-site, and perform classical repertoire alongside native 3-D formats in the same concert. This paper describes the technical design of the Virtualmonium, assesses the success of the prototype in some preliminary listening tests and concerts, and speculates how the instrument can further composition and performance practice.