Hi Jambourgie,
I haven’t really tried this in a long time, but If you have 2 in/4 out on your Safire 6 (which I guess you do) you probably need to route your playback output (to the external gear) to 3 and 4 out, so you’re not sending both your playback output and the input monitor to 1 and 2 out, which I would think is causing the feedback. If you’re on Wavelab 6, you could also check out the external gear plugin in the manual.