There has to be some kind of crossover or you will just destroy the tweeters and probably the midranges, too. There are head units with built-in crossovers, and stand-alone crossovers. Back in the day I would have fabbed passive crossovers, but there is math involved in that and you have to know a lot of details about the drivers. Make sure the crossover is 3-way, and that you have 6 channel capability. Really the only channels that need a lot of grunt are the woofer/subwoofer channels, typically they need 3-4x the power of the other drivers, give or take. If you got one big 2-channel and a smaller 4-channel amp, you'd be set.