I was just trying to understand how many line inputs I needed :) I currently have a Toneport UX2, which as separate line inputs from the microphone inputs, but Gearbox doesn't run on my Mac anymore. Also, it wasn't clear to me from the manual whether the UX2 knew how to monitor the line inputs and merge with the audio from the computer (youtube / itunes). The "Line 6 Audio - MIDI Devices" panel on the mac doesn't talk about any of that.
I was thinking of upgrading and trying to figure out how many inputs I needed. A Focusrite 2i2 only has 2 inputs that are combo xlr and 1/4 inch, but normally I'd connect microphones to that. So I was trying to understand if I needed extra inputs or not, because I don't want to unplug my microphones every time I want to connect the HX stomp.
When I looked at Focusrite 4i4 and the MOTU M2 / M4, their documentation talks specifically about routing line inputs to the analog outs / studio monitors, with very close to zero latency. They also talk about being able to control the mix between the inputs and the computer (the MOTU even has a physical dial for that). So that sounds like the right solution, instead of sending the audio from the HX stomp via usb to the computer and then back out to the audio interface. It sounds like I just need those extra cables, and I don't need an aggregate device.
I think that the latency is going to be lowest if you connect the studio monitors directly to the HX stomp, but if the audio interface supports direct monitoring like the Focusrite / MOTU / every other modern audio interface, then running a cable should be fine.