We've been asked this question several times before so we contacted a local QSC shop and borrowed a KW181 from them. We can best explain the difference by showing you the comparative measurements. The measurement files are attached below.
In order to get a 6dB increase in SPL you need to double the number of boxes. Given that the full-power measurements show the SSP118 has a 12dB output advantage over the KW181 on either side of 40Hz, you can calculate that you would need 4 KW181s to equal the output of one SSP118 as frequencies go below 50Hz.
Although the KW181s are cheaper individually, to get similar output (where you want it from a subwoofer) would cost twice as much and involve transporting 4 times as many boxes.