head unit - pxa701 - amp - passive - speakers.........correct??
this is normal, for ppl who dont have enough amplifier channels to run full active, maybe they just have a 4ch amp, so can run fronts & sub only.
so in this case, HU xover set to full range or off, depending on your menu. pxa only use front & sub. aiseh cant remember the pxa menu exactly but speaker select component and not sure use front 1 or 2, and of coz just have to set front highpass around 63hz, front lowpass = off, sub lowpass around 63 also and sub highpass off.
output wise its best to get the system scoped and then only can set the gains properly.