I'm late to the party on this response but it warms my heart to see Ripol designs gaining popularity. And it's especially interesting to see the Ripol concept integrated into a full-range speaker like this one.
I'm thinking the woofers' response would be the ultimate limit of the Ripol's frequency response but its chamber resonance would be a much lower, practical limit if one wishes to avoid it by choosing an octave lower crossover point, rather than a notch filter.
I use a pair of Ripol subs which blend seamlessly with my DIY hybrid ESLs, which also have OB mounted woofers.
I cross them in at 70Hz to avoid the chamber resonance (a nasty-loud peak at 299Hz).
Not everyone appreciates Ripol's because they don't hit you in the chest like a sledge hammer but I LOVE them.
They are simply the cleanest sounding, most non-boomy subs I've ever heard.
I'm looking forward to seeing some photos of your finished speakers!