creating a loopback in REW with MOTU M4 and WiiM Amp Ultra. No line out, but... subwoofer out?

[Shrek6]

I have a Dayton EMM-6 mic (XLR) and a MOTU M4 for recording audio. My system is a Wiim Amp Ultra connected to my TV via HDMI. So I have the M4 as my input device, and the TV as my output device (which just goes through the WiiM), and on my MacBook running REW, I have HDMI going to the TV.

When I calibrate my "sound card", presumably I need to create a loopback that involves the WiiM sending a line output into the M4?
The issue is the WiiM doesn't have line outputs.
But I did see that there's an option in the WiiM app to disable the subwoofer cross-over, making it effectively a full-range mono line output, which I could then send into the M4.
Will that work?

 

[staticV3]

When I calibrate my "sound card", presumably I need to create a loopback that involves the WiiM sending a line output into the M4?

Correct.

The issue is the WiiM doesn't have line outputs.

That's fine.
Soundcard calibration loses nearly all value when acoustics are involved, which are orders of magnitude more chaotic.

You can continue without and lose no precision, as both the WiiM and Motu have already been confirmed to be ruler flat in frequency response.