Hi Bray,
Maybe by "all the way down," the person meant to remove the sub's internal crossover or bypass its action, which is what you want to do to avoid "cascading crossovers". Since the A/V receiver's bass-management is already performing the crossover duties, you don't want to add a second crossover--holes and suckouts in the bass may result.
Some subs have a dedicated Bypass switch (a nice feature) but most sub crossover controls operate clockwise, with the upper frequency limit occurring with maximum rotation of the control or switch. That results in more or less the same effect.
Regards,
