I will make the measurement , I'm courius to see if in my case the rear wall is responsible for the drop under 50Hz... In the meantime I've read your project, very interesting! And how you judge the sonic quality of the dsp? I prefer go passive because I've read some negative opinion about the sound of the dsp module but I've never heard one by myself...
I have a MiniDSP and an active XO (system is biamped). I used the MiniDSP to model the XO before building the active XO (Rod Elliot P09 kit). The FR has been measured in room and the P09 overlays the MiniDSP almost exactly.
Playing the TT there is a difference between the MiniDSP and P09. The P09 has greater depth and instrument separation in the image, it's not a lot but it's there.