Integrating Subwoofer with ProAc Speakers.... HELP

I recently bought a REL Strata 1 to match my ProAc Tablette 50 Sig speakers. However I need help as to the cut off frequency I should set my sub at... Some responses from different sources:

- ProAc technical person says no such information is available. That is quite unbelievable!
- The local agent claims the setting at 80Hz. That is too high accodingly to REL agent.

Can anyone provide some light in this? Thanks in advance!