Splits dont have bass, thats what splits are for, so you have splits, just hign freq and then u have ya subs for the low freq.
the crackling you hear is whats called low level distortion, its because you are serverly underpowering them, and when you crank up the head deck to get it loud you are maxin the built in head decks amp and its only putting in a fraction of the power that the tweeters want, i hope that makes sense, its hard to describe.. but of course you could still have dodgy connections or they could be stuffed .. to check them out its best to remove the mid bass speaker and put your ear realy close to it and gently push the cone in and out with your fingers and listen for a scratching noise, it you can hear a slight scratching noise they are stuffed ..
Trust me it will be worth while to but an amp, even a cheap one. I dont know if you know much about car stereo's but splits are used only if you are running a sub, splits dont put any bass out at all that why you use a sub .. if you just have your head deck and you dont have an amp and sub you should be running 3ways, for example have 6inch 3ways in the doors and a pair of 6x9's in the parcel shelf .. you should pack the splits up and put them aside for when you decide to get an amp and subs and buy the 6inch 3ways and 6x9's, you will be happy with the sound i bet ..