Amp sub crossover and a sub crossover

My McIntosh preamp has a switch for the sub to cross at 80 hz---and my Velodyne has a variable crossover. So if I want to cross at 80hz. Should they both be a 80hz???Where should the Velodyne be set??
Set the Velodyne crossover to its highest frequency setting, letting the preamp take care of the crossover duties.
I agree with Altbrewer,

By trying to use two electronic crossovers at the same time you only risk having more phase issues and lesser quality sound.
Agree to set the sub's crossover as high as possible. Some subs have a "Disable" switch on the crossover so use that if yours does.
I agree with the others. I'll add that having two 80Hz low pass filters in series would result in a bandwidth that is significantly less than 80Hz.

Also, according to the review of the sub that is linked to in your system description it looks like the sub's crossover is indeed defeatable, so Bob's suggestion appears to be applicable.

-- Al