it was showing huge modes at 50, 100, 200, 300
Hmmm.... And you live in Australia, where power is 50 Hz....

I'm thinking "Mains Hum".... Did you use a properly wired cable for that? Everything correctly grounded?
Just checking, but did you have the U89 on omni for those tests?
OK, looking at the REW data, there are several puzzling things there: First, either you didn't do the calibration correctly against a sound level meter, or your room and system are amazingly, astoundingly quiet! According to your graphs, you did those tests at about 55 dB, meaning your RT-60 measurements went down to -5 dB

I'd re-check the calibration of REW against an accurate SLM. You should do the tests with each individual speaker giving you around 80 dB on both the REW meter and your SLM, so both together should give you about 86 dB.
Next, the readings from Left and Right speakers are very, very different. Almost like it was two physically different speakers. There's a difference of around 4 to 15 dB at all points across the spectrum, with L being higher in some places but R in others: Perhaps something changed in the room between doing the L and R readings? Maybe you yourself were in a different place, or you moved the sofa, desk etc.? Another curiosity: summing the L and R SPL curves does not match the "Stereo" curve. Or rather, it does match in the low end, up to about 2 kHz, but above that it just goes wild. I would try to figure out why the two are so different. The L and R curves should be at practically the same level, showing more or less the same thing (except for minor variations due to asymmetry in the room), and the reading with both speakers on should be consistently about 5 or 6 dB louder across the board.
Next, it looks like you overdid the baffle step compensation. On all the graphs, there's a consistent drop of about 4 dB below 120 Hz. How do you have the controls set on the rear of your speakers? Are you using a sub? Are you using anything else that could be shaping the frequency response, such as a graphic equalizer, or a crossover?
The good news is that the room seems to be behaving very nicely!
- Stuart -