It pays to measure ...

Eric, I can't help with your main problem, but I wanted to point out that your processor supports an older version of ARC. It is available here: https://www.anthemarc.com/downloads/index.php
I can't say that it will be better or worse than REW, but it is made to work with your AVM 50. Two very different approaches! However, at least current ARC works only with Anthem's own microphone. It will not work with a UMIC-1, for example.
Have fun!

Erik,No idea if this will help at all but here is a detailed review of ARC by Christian Punter he's pretty thorough.
https://www.hifi-advice.com/blog/review/loudspeaker-reviews/anthem-room-correction-and-martin-logan-...

Hey @mike_in_nc

Sorry, my friend, but sadly, I have the AVM 50, not the 50V, nor the 50-ARC.

Mine lacks the internal DSP power needed.

The issue I’m having is with the manual crossover settings. Setting the center speaker to small, and setting the crossover point at 100 Hz seems to have had no measurable effect, except for the C signal to be duplicated at the subwoofer.  Or at least, that's what it seems like. Subwoofer also seems to be without a LP filter unless I put it into miniDSP. I've double checked the settings, and they all seem right. 

Weird, but compensated for.