So the deal is that the wattage of a speaker is what it needs to run. And the wattage of the amp is how many watts it can provide.

Yes.

What does the wattage of the speaker imply? That is, assuming I have an amp that could handle either, what advantages would I have in picking 75 Watt speakers versus 50 Watt speakers?

Speaker wattage is how many watts you can cram into them before all the smoke leaks out of them and they stop working. 75 watt speakers can accept more wattage than 50 watt speakers, and thus make more noise before they stop making noise permanently.

Let me see if I can summarize all of this succinctly...

Amplifier Wattage: The more you have, the better sound you can get. Just think of it as the maximum that the amplifier can produce; you don't use all of it, and the less you use (percentage-wise) the better your sound.

Ohms: Smaller Ohms (lower number) means less resistance. Less resistance means it is "easier" for the electricity to go through the speakers, thus they will draw more wattage and play louder. Think of it like a drinking straw -- less resistance = larger diameter. Downside: lower resistance/higher wattage means less clean sound. Top quality competition cars (those competing for sound quality, not for maximum decibels) frequently use 8-ohm speakers.

Speaker Wattage: See second paragraph, above.

Sub Enclosure:: A ported box is louder, particularly at the resonant frequency determined by the area and length of the port. This is likely to give an uneven frequency response, with a "hot spot" at the resonant frequency. A sealed box gives better sound quality, with a more balanced response across the frequency range. It is not necessary to have a direct air path from the trunk-mounted subwoofer into the passenger compartment... IF there is an acoustically transparent path from the trunk to the passenger compartment, such as through rear seats with fabric upholstery.

Channels:: The more the better. My ShoWagon (currently under construction) has eight channels for the front/rear stage, plus four more channels (bridged stereo) for the subs. I highly endorse... hmmm, was it rockstar? suggestion of a separate amplifier just for the subs. It gives many more options for tuning, and relieves the load on the front stage.

Well, this is already a much longer post than I had planned, so enough already. Hope this puts things into perspective.

tanstaafl.