I have 6 mics plugged into my FireStudio Project audio interface. I like the zero-latency monitoring available by plugging headphones into the interface, but I can't add simple effects, like reverb. If I want to hear effects while monitoring, I have to plug the headphones into my computer. Is there a way to apply effects directly on the FireStudio, so I can still use its outputs for monitoring? I read a little about sends and returns, which (i think) route the audio through an effects device, but is there a way to do this without additional hardware? What's the actual term for applying effects directly on an audio interface?
You cannot apply audio effects with zero latency and no external hardware.
Your Presonus has insert connectors for channel 1 and 2 on the back for external hardware. That is probably what you should use.