Amp sub crossover and a sub crossover

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.

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.

Regards,
-- Al