"They" say that a TH118 can equal two or three boxes such as an SRX728. If an SM80 can keep up with three TH118, this would mean that it could keep up with six to nine SRX728. I find this to be highly implausible.
The term "keep up" means very different things to different people.
If you mean that keep up means to have the same max level-then that is VERY different than what I said-which is I would want at least 2-3 TH118's per SM80 to do loud rock bands.
Now maybe I like a lot more bass than others-but I was talking about situations in which the sub are run a good bit ( like 15dB ish) hotter than the mains.
If you are not running the subs (and not just the "alignment" but more about the mix ( lots of kick-deep bass etc) as hot as I sometimes like (music style correct), then you won't need as many subs.
You also have to consider what you mean by "keeping up with other subs". At what freq? At 80hz it may be one thing-at 35 Hz it may be a completely different number-based on the ACTUAL freq response of the cabinets.
As with all things audio-you CANNOT dumb things down to a single simple number. That is why I said 2-3TH118's when doing loud rock bands. If I was doing lighter types of music-the number would be different.
I am sure different people milage will vary-just like driving differences make on real milage.