Set the DR2d mic input sensitivity to low-gain. Turn mic-power off (the Sound Pro's box will be providing power to the microphones so it is not needed from the recorder's mic-input)
Line-in @ 100 provides approximately the same input sensitivity as Mic-in @ 67 (if mic-input is set to low gain in the menu).
Start by using mic-in @ 67 or line-in @ 95 + no attenuation on the battery box (AKA power box / microphone power supply). If you find levels are peaking or uncomfortably close to clipping, switch in attenuation on the battery box to get the levels down to a comfortable range. Done.
However, if levels are too low, use mic-in and raise the input level on the recorder above 67, in combination with no attenuation on the battery box. Done.
If levels remain too low with the mic-in level raised all the way to 100, switch the mic-in sensitivity to medium-gain and try again.