This seems like a nice monitor, but I actually think it's a good example of a case where a sub isn't going to play much louder. Looking at the distortion and compression, it tops out at around 96ish dB @ 1m before
compression and
distortion start to become a serious problem all the way up to 200hz. 3rd harmonic in particular is already over 3% at 96dB. Since these graphs are tested with sweeps(ie only one frequency playing at a time), adding a sub would not change the results of these tests, you'll still be limited at 150-200hz best case.
Considering the price and 5" driver size this performance is quite good though. If you compare to the much more expensive
8331A which also has a ~5" woofer, yes it performs better at 96dB(especially
distortion, 3rd harmonic is much lower!) but not by THAT much. You might get another 2-3dB out of it before it reaches its limits.
FWIW I think if you're listening at more than 2m you should just be looking for something larger than a 5" woofer, 6"-8" minimum. Below 2m the output isn't going to be a huge problem.