First of all, kudos to the OP. GREAT post. Secondly, if whoever recommended a sealed box means one with a sub enclosed entirely inside a box, I've always heard the exact opposite. That box type is for long, dragging bass like hip-hop, etc. He wants quick, punchy bass I assume. Of course, I could be completely wrong, but that's what I've always heard.
Maybe thinking of a bandpass box - which does have a narrow response for certain frequency range, but the rest of it just blows through (hence the name).
A sealed box (and a properly ported one, ..) augments the the motor's response by keeping it under control at the limits of excursion - and tends to bump up the 60Hz range which is just the ticket for kickdrums, punchy sounds etc. Also, can be smaller than other box types and still tuned correctly.
OP you might do well to just improve your doorspeakers' environment; foam cups, kickpods, damping material, rigid baffle - that'd probably fix you right up.
You can add a lightweight 8. Car guys almost always go overkill on the woofers. Ya don't need a monster.
. If you want to have 2 subs, you can actually run a left and right sub, but you need a 2- or 4-channel amps to run stereo subs
Low frequency sound is omnidirectional, if you run multiple woofers (especially if they're making different sounds at the same time, as stereo) you'll just make a mess. Everything that resonates behaves like a speaker. Fewer points-of-origin is best. If you have to run multiples for some reason, use the same signal and amplification
Is it better to have a 5 channel amp?
Not particularly. A 5-channel amp is probably a Class A/B amp, and be less efficient for subwoofer duty than a Class D amp would.
listen to other peoples' cars
Here's the bitch of it -- your cars are the same, speaker's the same, but your ears are different.
Every possible configuration will sound different to each person because of those microphones - they're as individual as fingerprints.
