what head unit you using? basically the HPF LPF is to set the
frequency where above it, goes to your speakers, below it goes to
your woofer. my setup is mbquart 4" coaxials and bazooka 8" tube
only. previously my head unit dont have HPF LPF, and i used a SPL
pre-amp with sub crossover. as a result, my sub only cover up to
say, about 80Hz, and the coaxials can only comfortably handle
100+Hz and above. hence there is a band of freq in between that is
'lost'. when i try to turn up the bass for my coaxials, distort
because only 4". cannot produce much bass. now with the HPF LPF, i
set at 120Hz, below go to sub, above go to speakers. wonderful. can
play loud volumes, very full sounding.
actually i thought of changing to 6.5" components to handle more
mid bass, but after realising what HPF LPF all about, solve all my
problem. basically set your HPF LPF both to the same frequency.
above this will go to your speakers, below will go to the sub. see
to your liking, play around. 
