hoth, I'll bet you plugged into either the XLR or the 1/4" in the Neutrik combo jack on the front of the FW410. Either one of those will try to run the signal through the FW410's mic preamps. What you need to do is on whichever channel (1 & 2), you need to flip that Mic/Line switch, and then plug your Grace pre into the 1/4" input on the back of the FW410.