Other than the ads I've seen, I don't know anything about Soundbok speakers. While technically not a subwoofera, has anyone taken a look at them as an option to give more bottom end to an S1 centric PA? They claim a long battery life and durability along with thumping bass.
The Soundbocs is a full range speaker and is (if their website is to be believed) capable of 126dB SPL. At 16dB SPL louder it would pretty much make he S1 Pro redundant.
Bit the only one I can find on sale is a used example on Amazon, the retailer DV247.com list it as 'no longer available' so it sounds like it my be discontinued.