Hi Zero,
I don't think the routing is too complex comparing the mightiness of the saffire 56 ;-) I use the smaller 24 DSP; probably the two interfaces work in a similar way - the software seems to be nearly the same. I connect the lineOut of the keyboard to one of the first two inputs (combo socket XLR-TRS) and switch to instrument gain; as I understand the saffire 56 has two dedicated instrument inputs (TRS) at the front. So it seems to bee even easier for you.
Good luck,
Wolfgang

Core2 Quad, 2.33 GHz, 4GB, Win 7 Ult., onboard
P4, 3.0 GHz, 3 GB, WinXP, focusrite saffire 24 DSP