Actually, I'm not following you...sorry
I first wanted to establish that when you remove the speakers and SPL meter from the setup and simply use a loopback cable, it establishes whether the Soundcard and computer works with REW.
If you're able to get the proper levels on a Soundcard calibrate setup, then you would check that you can measure a cable (after first setting up correct levels) and get a flat line response (using the previously created soundcard.cal file).
When setting up the Soundcard cal routine, the output and input REW VU meters should be in the Yellow at about -12dB. Are you able to accomplish these levels? Like below...
VU meters.jpg
Then while leaving the loopback cable in place (after you've finished the soundcard cal file routine), can you use the Check Levels routine and get the meters again to look like the VU picture above.
Below is my Check Levels settings to accomplish that. Can you show me yours?
Check Levels.jpg
brucek