If there is material in the 40-80Hz range, it's because it has been mixed there. Maybe it's a bad mix? Did you try with different source material and got the same effect?
In any case, setting the xover at 80Hz will redirect this info to the subwoofer so it would still be there. Can't really understand how that effect voice clarity. That puzzles me...

Bruno
M80s/VP180/QS8s/EP600/AVR-890
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"The problem is choice..."