I see you got it figured out but your question probably baffled folks.
Hardware synths would not normally be involved in outputs unless what you were trying to do was send the hardware synth to 3/4 for separate monitoring. That's not a great idea as that would involve the signal passing through the interface and latency would be an issue.
A hardware synth has an analog output that would either go to a mixer or to your interface INPUTS.
It would then be monitored directly at the interface without the latency.
Someone with only a 2 input interface would be better off using a little mixer in front of the interface so your not always swapping cables. You would then use the mixer for monitoring directly without system latency.