Being the installer here...
The premium system consists of 4 speakers. each speaker has a 5 1/4 midrange and a 1 in. tweeter. they are internally crossed over, so even if you upgrade, you get zip for bass. The amp for the 4 speakers does drive the signal for the sub amp. If you replace the sub with another, the stocker is only about 60 watts or so and I think the sub amp is about 80 watts if I remember hearing right. You can put a 4 ohm sub in as well. They use the same amps in the mach system, and they run at 4 ohm, all day. IF you are putting in a new radio, you will need one with preferably 2 pairs of rca preouts. The reasoning behind this is if you wire the speaker level inputs into the small black plug, and run it through the amp, you will only be able to get about 1/3 volume on your new deck, and you will be risking overdriving your factory system. My solution has been to go to radioshack, and wire rca ends onto the harness for the car, and just use the pre outs from the radio. It sounds much better and much cleaner. If you get one with sub control, you can run a pair back and with the combination of a y adapter, and cutters, cut and tie into the signal for the sub amp and have total control over it. This way, you can also tune and tweak it to its fullest. Also, I beleive the factory amp for the 4 speakers is only rated at about 15-30 watts per channel. Hope this helps. This is all common knowledge in my book. Any other questions, let me know.