Why would you want to use the 1/8" input? The XLR line in is the way to go, and if you get a modded unit, you benefit from the upgrade using the XLR line as well as mic inputs. If you want to run a preamp with an unbalanced line out, you just have to get a pair of the Neutrix male XLR to female RCA adapters to plug into the 661.
I run a pair of the Sound Devices MP-1 mono preamps in front of my WMOD 661 and love the combo.