Look, with an unregulated power supply, as you half the ohms or resistance of a speaker, you double the power. So, for instance, if the amplifier is stable at 1 ohm at 600w, then at 2 ohm it would only output 300w and at 4 ohm only 150w. So, if you can't find an amplifier stable at 1 ohm, then your going to have to wire it up to the amplifier at 4 ohm, instead of 2 ohm with the other speaker.
So... your either limited to amplifiers stable at 1-ohm or going to have to buy some monstrosity and expensive amplifier to supply the 400+ wattage at 4-ohms. Thus taxing your electrical system and maybe requiring the big 3 upgrade. See what I mean? We went from getting the other sub to getting an expensive amplifier..... big 3 wiring.... aftermarket alternator.... it gets expensive quickly.
As for the tweets, hook them up to your head unit or something... You don't really need them. The components you bought are going to have more than enough high, I promise. Your weakest link is going to midrange-midwoofer. Not high or low low.