I have multiple USB mics that I want to record with, each show up as a single input channel and a stereo output channel. If I create an aggregate USB device of those using the Audio MIDI Setup app that new device doesn't show up in Studio One under "Recording Devices" ... but it does show up under "Playback Devices" - and only if you chose that will that aggregated audio device populate in the "Recording Devices" drop down. In fact, it will be the only option listed there. If I want to output to the built-in output device (headphone jack) it's impossible to do that ... unless I add that into the aggregated device too.
Even more concerning, if I create an aggregated audio device that only has inputs (built-in and the mic jack on my docking station) that device doesn't show up in Studio One at all.
I opened a support ticket on this and was informed that since I found a workaround (add an output device to the aggregate) that my problem was solved. Clearly there's a bug here, if not a feature request. On the same MacBook Pro I've got Ableton Live installed, and it's able to treat the aggregated device as I'd expect - I can choose the built-in audio output and aggregated device as input.