I spent the last two weekends replacing the stereo system in my Mustang (most of that time was spent making the custom mounts for the tweeters and the DDX7015 head unit). I'm an acoustician/physicist/electrical engineer by trade and a serious audiophile and let me tell you, the Mach 460 isn't all that it's cracked up to be. Some things I learned: The door and rear "super sound" speakers (not the tweeters) are rated at 6 ohms. That's been stated before on this board so it should come as no surprise. What killed me was how these drivers were wired. There are two amps that drive these four speakers, each amp driving two speakers in parallel. The parallel wiring drops the impedance seen by the amp to 3 ohms which is why you can't just pop 4-ohm aftermarket drivers in their places since this would make the impedance 2 ohms which is a significant increase in power required of the amp. Ok, "what's the big deal?" you ask. When I skimmed the circuit schematics I just assumed that each side was wired in parallel, i.e. the left door speaker and left rear speaker are connected to one amp and the right door speaker and right rear speaker connected to the other amp. Nope. While I was trying to figure out a way to use the factory wiring for my new system I saw that the front speakers are wired together and the rear speakers are wired together--you are not getting stereo from the bigger drivers! I don't know how it's wired in the head unit but I'm assuming the two amps are driving the same signal, i.e. mono signal. Unbelievable! Don't believe me? Just look at the schematics that you can download from www.flemworld.com I recall seeing somewhere that the crossover frequency for the "tweeter" and "woofer" is around 300 Hz. It's low enough so that they can ALMOST get away with wiring the speakers like that (sound is ALMOST omni-directional at that frequency). However, for those of you who replaced only the drivers in the door sails beware. Aftermarket tweeters normally require high pass filtering above around 2.5 - 3.5 kHz (as my Infinity's do) so you would be putting incredible stress on your tweeters by driving them with the stock head unit which would be sending signals down to 300 Hz which is too low for a real tweeter to handle. Besides, the balance between the midrange/woofer and tweeter would not be right (you may be getting too little or too much highs). Needless to say, the sound of my new system is at least 1000x better than the stock system and I didn't even put in outboard amps! It's hard to believe that Ford and other car manufacturers would install such crap stereos in their cars (I just saw the pics of the 2005 Mustang Shaker speakers in another thread). It would be so much easier for them to simply OEM some regular head units (with built-in amps) and some decent speakers and install those rather than come up with some convoluted setup like the Mach system. It truly is convoluted!