I honestly didn't get this part.. a speaker amplifier will use a fraction of what a subwoofer amplifier will use (when driven hard), so I don't see how your pulleys are a limitation of any kind. In any case, you will probably find that almost _any_ speaker will get loud on head unit power. The issue is with distortion. With a dedicated amplifier you know you shouldn't hear any amplifier generated distortion. With head unit power, the signal can easily get distorted before it is converted into a mechanical wave. A discerning ear will identify that something is off. In any case, if you want speaker with good sensitivity, consider Morel Maximo. A lot of people said they take very little power. Of course, they would still benefit from a good amplifier.