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.