The SLAPS M12 specification says it's Fs is 25.5Hz with only the bolts installed, without the mass.I didn't use the weights. It was too low. I like to hear my music and feel it. Read somewhere without mass it was 30hz.
Your box looks to be ~1 cubic foot (or more) per woofer and M-12, putting the Fb (box tuning) below 30Hz with no added mass (the "0" vertical line) if the tuning graph is correct.
So...Something was off I noticed after a few weeks. The front radiators were the V1s and the bolted ones were V2s and the TS parameters were different. Off...being a little phasing🫠. All V2s now with added ring mounts so they fit right and man....it is absolutely ridiculous the damage this tiny 15 box is doing❤️ These things still on sale on EQ site. Welter you are also spot on. The V2s I last bought had the booklet and they are at 24hz. I never go that low in frequency and can't understand why I never did😎
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Love it....f n earthquake!