Speakerboxlite prediction doesn't match REW measurements at all

So I designed a subwoofer in SBL, basically a 325mm cube with an Dayton Audio Ultimax UM10-22. The graph in SBL shows 40Hz at -3.8dB and 30Hz at -6.2dB. Now I built it with a Monacor SAM-300D and measured it with my UMIK-1 and REW, and it measures 40Hz at 71.4dB and 30Hz at 55.9dB which is way off from what I was expecting. What am I doing wrong?

As I can see you have a very high slope, maybe you turned on subsonic or have any crossover in your system?

There's no configurable subsonic filter in the sub amp, and I've set the crossover to 200Hz (the highest setting). I've ordered a different sub amp now, Dayton Audio SPA500DSP. Hopefully I can tune it flat using the DSP.

You designed the box for a Dayton Element but built it with a Monacor…?

Just the plate amp... I didn't think that that should make a big difference in the box design other than the cutout.

I've put the Dayton Audio amp in there now and with the DSP I managed to flatten it out a bit, but it's still not great. https://i.imgur.com/2hd3eXX.png

So just a curious question did you add any bracing and account for the displacement if so

The 300D has a built in bass boost at 40 Hz. Depends on how they implemented it. If it's a high Q high pass it works as subsonic filter as well. That might be the reason of the steep slope and the reduced output at 30 Hz. 

This isn't a Nearfield measurement, right ? So IMHO you are measuring the room as well at such low frequencies.