The Tau Factory setup has two MIDI outs. We have just released our first, experimental 2.0 which in the Pro version subscription does include Workbench, which would allow you to build a setup with just MIDI outs if you wanted to (as many as you like in fact, there's no inherent limitation), but this really is 'experimental' as yet and I wouldn't recommend it to someone new to the Eigenharp at the moment. Give us a couple of months to get it into 'Stable' unless you're feeling very adventurous.
Having said that, Massive is also loadable as an AU or VST. There are 4 AU/VST slots in the Tau factory setup. Using Massive inside one of these will give you considerably better performance than running it in an external host or standalone - you'll get considerably lower latency for a start and in general there will be less CPU load with the high data rates the Tau can generate. Is there a specific reason that you want to run Massive standalone?
John