Looks like you're gonna go in the line inputs on the back of the Fireface. That way you're gonna bypass the mic preamps. You can select -10 or +4 on the unit.
The GRNV will give you either. The -10 output from the NV (according to the manual)
"...is derived ahead of the output transformer, so it has a bit less of the 'iron' characteristic. You might try this output if you are going for a very clean signal. This output won't put out enough to drive a +4 dBu input to full level, though, so it won't always be useable."
The line inputs on the back of the Fireface are TRS, meaning that they're balanced inputs. In my experience, without 100 feet of cable, it won't matter whether or not you use a balanced input or not. Some may disagree on this. Personally, I'd prefer the post-tranny output from the GR, but the cool thing is that the Fireface will accept either output fromt the preamp.
What you're gonna need is an XLR -> TRS cable. If you're halfway decent at soldering, you can whip one up in five minutes.
Sounds like fun!
Edit: The GR NV manual also says that the 'loading' button on the front panel will activate a termination resistor. Anyone know what the value of this is?