The key is correctly matching the speakers to whatever power level (watts) is used. If you have speakers that can handle 300 watts, and only power it with 50, it will never sound good, the speakers won't "move" enough air. You'll turn it up louder, and while the wattage isn't there, voltage is, and you'll soon burn out the speaker coils. And the opposite is also true. Over powering speakers will also blow them out. I've built many lower powered systems that sound great. You really only need amps if you want your ears to bleed.
The other thing I'd point out is to have a properly sealed speaker cabinet. That doesn't literally mean a cabinet, though. In the case of older Bugs, cutting a board to fit over the package compartment is the proper thing to do. But sealing around the edges is the important part. What happens is that you have sound waves coming off the front of the speaker, and opposite waves coming off the back. They will tend to cancel each other out. I always cringe when I see some lazy a$$hat put a pair of speakers just laying on the rear window shelf. It sounds crappy, and in an accident, they become weapons of head destruction. Don't be that guy.
