Adding sub to Sonus Faver Guarneri Evolutions...need advice where to set crossover on sub


As the title say, I recently pulled the trigger on a Rel T/5i sub to pair with my Sonus Faber Guarneri Evolution monitors. Ultimately when funds allow again I will add another sub. For now, I only have 1.  I have a small room that is 9x10x9 with the back wall only being 3’ tall, above that the room opens up to my family room and kitchen. I listen at fairly low volumes, no more than 75dbs typically. The Evolution speakers start to roll off at 72hz and its -6db point is 43hz. Its -3db point is around 60hz. Any advice on where set the crossover on the Rel would be appreciated.
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I followed the setup procedure outlined by REL.  My B1 integrates well with my Auditor Ms.  My phasing is at 0. 
Your OP question is very difficult to answer. I would suggest bringing in the sub at about 50Hz. This is where the SF's tend to get a little weak. However, the room plays such a large part, that experimentation ( and lots of it) is the word of the day, IMHO.
I have since repositioned by speakers and get my sub dialed in (I think).  In my room, I was actually getting good, flat sub response from 50hz - 40hz.  Once I spent some time moving my speakers around I get +- 2db response from 100-20hz, with a +4db hump at 40hz.  The mid bass and bass in my room is very tight and punchy.  Perhaps my hearing is not good enough, but I didnt find integrating my sub that difficult but I have never had "golden ears".