Crossover selection for subwoofer

From experience, can anyone tell me what is generally the best crossover setting for a REL sub in use with Sonus faber grand Pianos?
In general, it is not advisable to select a frequency higher than 70 Hz. because if you do then you will hear where the sub is situated.
You may want to check Harman's web site as there are a few technical articles by Floyd O'Tools team discussing the subject. I recall Richard Vandersteem talking about how a subwoofer needs crossing over an octave above the main speakers -6db lower limit, but I'm not sure about that. Hopefully this will get you started.
Happy Hearing,
I found that THX 80hz stinx.I have an EAD Ovation-8 and use 65hz.It is the point where my minimonitors roll off anyway.I would rather my mini's reproduce 80hz than my subz.