Four TH-minis powered at rated continuous level would have 3 dB more output than a single TH-118 (140 dB vs. 137 dB) above 50 Hz.
Below 50 Hz, the TH-118 would rule.
If the TH-118 is as loud as my Keystone sub which uses the same driver (and happens to be 45" high, like the OP is looking for) it would easily go louder than the SB1000Z.
You are correct.
The thing that is SO often overlooked is the actual freq response. Again you cannot look at a single number and get an idea of the whole performance.
For example at 40Hz (not all that low) you are looking more like 8 TH minis for each TH118. And lower than that the number gets even higher.
So when looking at subs, here is the "procedure" I suggest.
1: How LOW do you REALLY need to go. A good way to tell is to look at the actual freq in the type of music of interest.
2: How LOUD do you really need to go? Think continuous at a particular distance-then add some overhead.
3: THEN you start looking at sound quality-price etc.
If a sub doesn't go as low as you need it to-then it doesn't matter how good of a deal it is.
If it doesn't get loud enough-it doesn't matter the deal.
People will argue with me-but if you can't hear it-it doesn't matter how good it sounds (at a closer distance).
Physical size is another issue that must be considered. Let's look at the TH mini vs TH118 issue. At any freq-the TH minis will take up more physical space in a vehicle. At the lower freq- a whole lot more. So you need a bigger vehicle (possible more cost). They would also weigh more.
Also due to the impedance, you will need more amps to run the TH minis (more costs). And more speaker cables (more cost). More trips to the truck and back (more labor/hassle).
And finally the 4 TH minis cost a good bit more than a single TH118.
So the TH118 is actually the cheaper of the cabinets (for a given SPL). So even if you got a great deal on some minis (and the cost was the same) they would not go as low and be more hassle than a single louder cabinet.
This is not saying anything bad about the mini-if you want to put your sub in the front seat-the mini wins. If you have to carry it up stairs-the mini wins. If you are trying to hide the sub-the mini wins and so forth.
It is just another tool for the toolbox. Not that one is better than another-but rather better for a particular usage.