hu lp filter and amp lp filter

Pretty much go with the one that has the most flexibility.

As in

Variable > selectable > fixed

Turn your HU's LPF to its highest setting... then adjust with the amps LPF. This will give you the most flexability with cutting frequencys out of the subs. The

18db/octave filter wil be better than the HU's anyways.

-However if you like driver seat tuning, completely reverse what I said. The amp at highest point, and use the HU's. It might just not integrate very well with the rest of your system given the default freqs.

Set it to wherever it sounds best...could be between 70-90, wherever. You don't want to go over 100Hz as sound begins to get directional if you go too high, and you'll be able to tell exactly where the subwoofer is. Not to mention that most woofers don't like playing up that high anyway.