SabrinaX Sub-Crossover

First time posting. I recently got a new pair of SabrinaX speakers and paired them with two Martin Logan 1100x subs. The system together sounds incredible. The SabrinaX is rated down to 31Hz. Where should I cross-over the subs? I did it at 35Hz with a steep roll-off (4th order) but wondering if people with more knowledge have any recommendations? 

Thanks!