Multi-Sub Optimizer Reference (page 6)
Defining Filter Channels
MSO does not maintain a database of the various DSPs available and their capabilities. Instead, it assumes the DSP has a simple structure analogous to that of the miniDSP 2x4 with the four-way advanced plugin. The DSP structure is assumed to consist of a single bank of filters whose response affects all channels at once (called the "Shared Filters"), together with an arbitrary number of independent filter channels added by the user.
To add subwoofer channels, select the Config View tab. You'll see an initially empty configuration defined. Right-click the Subwoofer Channels node and choose Add Filter Channel from the context menu. This will create a new channel initially named "Sub Channel 1". You can rename it in the same way you renamed the measurements in the earlier step. Repeat this operation for as many independent DSP channels as you actually have in use for your subs. Also, even if you're not doing any DSP on the mains, you must still create a single mains channel so that mains measurements can be associated with it. To do so, repeat this procedure by right-clicking on the Mains Channels node.