ok you have 4 rca outs and are wondering how to hook up to a 5 channel amp?
you split to create a 5th channel
did you turn on the LPF/HPF?
also you will need to adjust the frequency it cuts at as most are adjustable
set your gain to no more than 60%...
if thats not loud enough...BUY A BIGGER AMP!
(your wasting your time and damaging your sub if you set it higher)
high level/voltage adapters will usually have some bass boost options and some even have adjustable crossovers in them....along with gains etc
6x9's are full range and usually tune out lower bass before they hit the tweets...adding another passive crossover before is a bad idea..as it just never sounds good or lasts very long...let your 6.9's and any other fullrange setups (component sets, 6 1/2" 's etc)
run at full frequency with no filtering
also some passives do not like running rca and line level inputs st the same time.
be very careful to ground any passive crossovers and powere'd crossovers as they are often a little crappy and can have some sound issues as well...
that being said high end equipment is MUUUCH better than most of the low end stuff so I'd go with my gut and get better stuff....
why not skip all this and get a better deck?
even $200 decks have sub outs these days