Well, since REW compensates for any inadequacies of the soundcard, the quality of the card doesn't matter. I would suggest using the RealTek, as it will have less complex features, effects, monitoring, etc that need to be turned off. It's a simple card to use.
Try using the RealTek (with it set to the 2 channel stereo mode).
Setting up REW only takes but a minute once you're used to it.
Anyway, try following these steps and see how it goes.
I'll also post the REW mixer for a RealTek card....
To go through these steps only takes a second......
Hook up the Line-out of the soundcard using a stereo connector at the soundcard end that breaks out the two mono channels (left and right) and connect both channels to your receiver AUX or CD input and put the receiver in stereo mode.
Hook up the ECM microphone preamp to the Line-in using a stereo connector, but connect only one channel (i.e. Right). Select that same channel in REW in the Settings icon.
1. Ensure the soundcard and ECM meter calibration files are loaded (the soundcard.cal you already created and stored). Be sure to select the soundcard as the input and output device in the Soundcard tab and select Line-in and Speaker(Line-out) as shown in the pic I Attached.
2. Run the Check Levels routine (in Settings icon) selecting either Subwoofer or Main speaker in the Levels pull down, with the SPL meter at the listening position and the dial of the meter at 80dB position (or whatever scale allows 75dB to be measured).
Ensure the REW output VU meter is at -12db and the sweep level at -12dB.
Set the Wave Volume at 1.000 and the Output Volume about ~0.500.
Then adjust the receiver volume control so the actual standalone SPL meter reads ~75db at the listening position.
Then adjust REW input volume (and the Behringer preamp volume) to end up with -12dB on the REW input VU meter.
3. Run the Calibrate SPL routine (in the meter tab) to match REW's SPL meter to the real SPL meter to ~75dBSPL.
4. Run the Set Target Level routine, which will set the target to ~75dB.
5. Run the Measure routine and set the End Frequency to 200Hz for subwoofers and whatever you like for testing mains (i.e. 10KHz).
6. Measure.................... (use a larger number of sweeps and longer length for more accuracy and less noise)
7. Using the Graph Axis Limits icon, set the vertical scale to 45dB to 105dB and the horizontal scale for subs to 15Hz to 200Hz (and mains to whatever upper limit you like - i.e. 10KHz)
8. When testing mains you need to add smoothing (left panel) after you measure. Use whatever amount looks the best for your needs. Don't use smoothing when creating filters - only when viewing a response.
Check Levelsxxxxx clear.jpg
Note: I should add that smoothing isn't normally used with subwoofer measurements, since filters optimized against a smoothed response will have settings that don't accurately match the room's modes. It's not usually necessary either. But for full range measurements, its almost impossible to read the plot without smoothing. Below is an example of a measurement taken from 200Hz to 10KHz. The first plot is the raw measurement and the next is with one octave smoothing. You can play with all the levels of smoothing to get the result you require. Usually 1/6th or 1/3rd is suffice.
no-smooth.jpg one-octave -smooth.jpg
brucek