The Paradigm subs, being ported, will have their minimum driver excursion right around the port tuning frequency. Every ported sub becomes unloaded below the tuning frequency and the driver will beat itself to death if high levels of input below tuning occur if not somehow protected.
Haven't paid attention to the ML subs in a while, but it's quite possible that rising distortion make a high pass filter the better option on those subs. No great thing to have sub 20 Hz extension if it's got 50% distortion once you get to audible levels.
I would never assume what a -3 dB point means unless I've seen a frequency response curve. And of course manufacturers will measure or predict the sub's output at relatively low SPL, and a big key is whether the driver has the excursion and the amp has the power to continue to output lower frequencies at the correct relative level once you start to turn up.
Lots of subs are advertised or specified at +/- 3 dB. Again, what this means can vary. A sub lke my REL that has a native FR curve that looks like the top half of a sine wave will be defined by the place where upper and lower frequencies are 6 dB below the peak of the FR plot. A sub like the Hsu 15H, which has a pretty flat frequency response in most of its passband, will actually be 6 dB below that flat response level when you are at +/- 3 dB. In effect, there's no plus, only minus.
Since manufacturers all specify differently (and some use DIN, which allows you to pump up the low frequency spec compared to an anechoic measurement because of anticipated room effects), you can't know what any manufacturer means without looking behind the specs, and you can't count on being able to compare different manufacturers' specs because they may have been arrived at very differently.