SabrinaX Sub-Crossover

The best/easiest way to answer your question is to measure the speakers in your room to see what they're actually doing response wise. It may state that they go down to 31hz in their specs but the actual in room response will be very different. You can use something like REW and a UMIK 1 to run a 20-20k frequency sweep and you'll be able to determine where to set up both your speakers and sub xover points

 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.

B

Hey there!

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.