The SabrinaX is rated down to 31Hz.
I think you are chasing a dream.
Even if you could get down to 20hz., you probably wouldn't hear it.
I'd just set up the Sabrina's to the best possible position.
First, try to optimize the placement of your main speakers and your subs. I like to use the AM Acoustics room mode simulator. Try to keep yourself and all of your speakers out of the lowest room modes.
I am of the school of thought that a higher crossover for subs is best, especially given all the feedback from listeners who started too low. Up to 80 Hz is a good spot. In your case, this will reduce the load on your amps significantly, hopefully making up for the low impedance around 2 Ohms for your mains.
Next, consider sealing your main speakers. This will raise the -3dB point, but also severely reduce distortion and excursion in the lowest octaves. Clean socks work great here.
Lastly, the DSP control over the subs will give you an unprecedented advantage over pure main speakers. Leverage it, but instead of striving for flat, strive for a descending slope from 16 to 80 Hz, about 1.25 to 1.5 dB per octave. Then set levels to taste.
My advice above assumes you will be using a high pass filter on the mains, if not, you’ll need to still seal your ports, and then use measurements to figure out the best integration point which will probably be around 40 to 60 Hz.
Undoubtedly someone will get huffy about my recommendation about sealing a speaker which is ported. I say it as a speaker builder, and remind you that the procedure is easily reversible, depending on the condition of your socks.
Since the topic of a high pass filter has been mentioned I figured I’d share this, as it may be of interest to some - particularly Ayre Acoustics owners. I own an Ayre VX-5 Twenty amp. For that amp, and a few other Ayre units, a replacement "high pass input board" is available. Ayre makes a tweak to capacitors on the board to provide a high pass at 100Hz at the amplifier input. So the amp can feed the mains at 100Hz high pass, and the other output on my Ayre KX-5 Twenty preamp can feed a pair of subs. Evidently it’s common for Vandersteen owners using Ayre equipment. I’m getting to know a pair of Wilson Audio Duette with a pair of subs and I have a pair of these boards. I haven’t used them yet, but I expect they will be very handy - especially since I very much like 2-way speakers paired with subs to dial in the low end. I should be able to play with any 2 way speaker and should get a nice benefit from taking stress off the amp, and the woofer of the 2-way. The fact of the matter is, the Wilson Duette sound so good without the high pass filters I haven’t been in a rush to install them!