Which way to go

use seperate amps that way everything can have its own crossover settings{put the 4 speakers on a 4 channel amp and the sub on a monoblock or 2 channel amp}If you say "I already know how", don't ask me a single question.

You said that the sub and the rear speakers are on the same channel? Are you saying they are hooked up on the same channels of the amp or that the RCA's on the deck are the rear ones that are feeding signal to the amp or did you just mean that the sub and the rear speakers are on the same AMP. Reason why I ask that is that if the rear speakers and the sub are hooked up on the same channels on the amp and the low pass is turned on then you are not using your rear speakers properly at all and are probally gonna smoke em off right away. You wanna set a hpf on your speakers in the car and use a lpf on the sub. I agree with the above statement though. Getting a second amp to run the sub and then using the 4 channel for your speakers is probally best. However if you are only budgetting for one amp then I would either hook up the rear speakers to the deck and use the amp to run the fronts or run both sets of speakers to the front channels of the amp and wire it in 2 ohm stereo with one front and one rear on each channel and then use the rear channels on the amp bridged to run the sub. There's my two cents. Also make sure the box is buit to spec for best results.double-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer