What are you losing? Some people cross the subs at 50.
You can turn one or the other off altogether. Usually the amp has finer control (variable) so turn the HU LPF off if you can. Otherwise set the amp as high as it goes and use the HU only.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's