Thanks to Endorka helping me engage my brain last night, today I took an hour out at lunch to go back into the studio to see if I could figure out what went wrong. Endorka suggested that it looked like a high pass filter was engaged, but there isn't the possibility of that in the UAD console!
I played some music and it didn't sound as harsh, and tried a few things just checking both speakers were playing equally (ie putting music in mono, panning from one to the other etc). Just to be safe I did a hardware reset on the UAD Arrow interface I've been using for these tests, and then checked I was still getting 80dB at the measurement mic from one speaker.
I double checked mic positioning and it did seem to have been knocked slightly, so I reset the position with my laser measure.
After that I did a quick test with everything and me in the room, and the graph seemed much more sensible. So I emptied the room onto the garden (of course it started raining as soon as all the tools were outside, and stopped as soon as I brought them back in), and repeated last nights tests. Once I'd done them, I knocked the mic off position and did a test, just to see if that might have been the culprit.
Todays readings seem much more plausible, are showing as quieter (although still about 4dB louder than the previous tests). I cannot explain what went on last night but things do seem more normal today. The test with the mic knocked out of place was very similar to these others, so that was not the reason.
MDAT file below, but we can see in the graphs that the ceiling treatment has smoothed out things quite a bit, but has accentuated some dips present around 400-500Hz. Not sure what they are but once the rear wall is done I'll use a tone generator to hone in on that area.
SPL:
Waterfall plot with just soffit:
Waterfall plot with soffit and ceiling:
Spectrogram with just soffit:
Spectrogram with soffit and ceiling:
You can also see on the reverb time graph below that the RT has reduced considerably. There is a peak in the reverb around 100Hz, though, which hopefully will die down with the hangers on the back... but could also be floor bounce so I'll drop an absorber there and test that at some point.