The best thing you can do is experiment with the different settings to see what sounds best. Pick one chapter of a DVD with good LFE and play it with the different settings. You will probably find that just because the speakers are rated down to 40hz, it doesn't mean they will still sound good at that point. Do you have a 60hz cutoff, or just the 40/80 (and presumably 120)? And if all else fails, depending on the type of speakers, you may be able to ask the manufacturer. I have been able to get a tremendous amount of advice on setup from Chet at Definitive Technology for my custom installation in my basement.