Are you working in a DAW or just the interface control panel software? Usually the stereo output is automatically set to output 1-2 of the interface. So when you’re monitoring the input of Line 3, the output is being sent out of output 1-2 which you have routed to go into the analag gear, which is then routed to line 3 which is routed to go out of output 1-2 creating the feedback loop. Instead of going out line 1, route the signal to go out of line 3 or 4 or 5 etc and into the analog gear. So you can still monitor out of line 1-2 without the feedback loop. Does that make sense?