well most of those digital processors are multifunctional ie they have crossovers and equalizers and also line out convertors as well as line drivers built into them and some even have time alignment. Most of them would be controlled with a laptop connected to them and they allow you to add more to your system than even the best head units allow - but as stated previously some of them can be very expensiveI'm assuming the processor would be like an equalizer?
your cheapest way out would be to simply add a line out convertor to your stock headunit and go on from there - there are varying levels of line out convertors - the cheaper solutions start at around R150 or so - all the way up to items like the JL audio cleansweep or even something like an audison bit one or even maybe something higher end like what maxxis is using ie the alpine c701 which even gives you multichannel surround sound in the car if you wan ( . ) as well
yes but you will then lose one set of speaker outputs because you will need to install a line out convertor to one of the speaker output sets ie either front or rear - what i would do is hook it up to the front outputs and use a 4 channel amp there - so you can have decent amplified front speakers - leave those rear speakers powered by the headunit and then add in a sub into the remaining two channels of the amplifier - thats how i would go about it - to be honest 4" speakers dont really cut it in terms of front speakers - most guys who are after sound quality would place the better speakers upfront - two reasons for this - the technical one is that you want the sound to seem like its a live performance on a stage in front of you - and then the second logical reason is that you paid for the sound and you sit upfront when you drive so you deserve the best speakers for you not for rear passengers who did not pay for the soundIs it possible to hook up an amp and sub to everything though?
- normally bass is loud enough to fill up the car but full range speakers not so all the time