It's a process for making all the waves from each driver reach your ears at the same time. That way you don't hear any delay between your highs and mids, for example. You need some sort of processing to do this. Here's a way to do it by ear:
Also because my amps all have their own crossovers that have been properly set, should I just let the headunit play through without the deck crossovers?
I would use the hu crossovers, and turn the amps to full range. First try 80hz for the hpf and 63 for the lpf. if the mids sound like they're in trouble, then go to 80 & 100.
you can always try a mix, too. the dc probably has a nice 24db crossover, you can use that one and use the hpf on the hu.