-3dB at 1.4Hz for a loopback is a good start for using the low frequency capability of REW.
My EMU 0404 has a DAC/headphone output with -3dB at 1.5Hz (up to 94kHz), but the ADC/input amp coupling is far more limiting with -3dB at 2.8Hz at 44.1/48kHz sampling, rising to 5Hz at 192kHz sampling (even though the AK5385 HPF -3dB spec is down at 1Hz for 44/48kHz sampling). With respect to using a compensation curve to flatten the loopback response, the compensation is -11dB and 140 deg phase at 2Hz, so some heavy lifting from the compensation when measuring below 2Hz. The bulk of that low frequency roll-off is from 47uF 63V coupling caps, which can be swapped out for higher capacitance parts (and even at lower voltage if phantom power is disabled), although it could be practical but quite risky to bypass them for dc response if the soundcard was dedicated to a particular testing setup.
