lol if your kapas and your focals sound like shit then somethings wrong with your HU. Take your car in and get it checked out.
heres the thing with the crossover theory jluv mentioned.
did you use a high level input from your rear speakers to you amp for your subs or does your head unit have an OUTPUT with RCAs for subs? If you went through your speakers, which you most likely did, then if there was a high crossover point on the stereo then the subs would sound like shit as well. It is also to my understanding (well, more an educated guess) that all the mach systems have all amps external and the stereo is just a dead head. if this is the case and you most likely used RCA inputs for you old subs, theres some fun shit you can do to bypass the amp.
as far as using focals and kapas w/o an aftermarket amp... thats just stupid. unless they were entry level focal coaxles (even then) you really HAVE to have a more powerful amp if you want any midbass from your door or rear speakers.
also if your system is set up correctly, you should barely hear your rear speakers. so putting your good speakers in the back is very very stupid.
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