Integrating Subwoofer with ProAc Speakers.... HELP

I would suggest listening. There is no right or wrong answer when your own ears likes what it hears. Nothing is better than your own ears. Your room is different that most anyone who can give you an opinion.

the REL set-up is fairly easy. the options once you get close based on their suggestions are very few (5 frequency clicks I believe?) ....you will find more differences with the placement of the REL in your room, than any particular frequency.

80 Hz is the low frequency effect (LFE) default xover setting in surround formats/receivers. I would begin w/setting the xover at 10-15 Hz below your ProAc's rated frequency response (which very well might be 80-90 Hz on such a small speaker as the Tablette), and live w/that for awhile. You might feel that an even larger gap sounds better. You'll come to realize that it often takes a variety of music over several days/weeks to really find what your ears tell you is the ideal xover point. Listening to familiar recordings, not just the "audiophile" or bass-heavy ones, is the best way to do this. And SPL meters can be useful but can also be more problematic in the long run. The REL manuals are excellent and easy to understand. If you don't have one, I'm sure that they are available.

Let your ears decide