I don't think that's the problem, I'm using the SPDIF output of the Realtek sound device on my motherboard and all effects are turned off. It's the same output I use for all sound playback in Windows. I don't use any soundcard calibration file in REW, neither for the Behringer or the UMIK.
Just to clear up, is your ECM8000 mic individually calibrated?
Why aren't you using the UMIK calibration file?