1. How to hook it up in the Magnum. Where does it tie in to the stock audio system? How is it powered? Does it need a crossover?
A car audio amp needs power, ground, and a remote turn on. The battery in the Magnum, Charger and 300 is located below the cargo floor in the hatch/trunk. You'll need a fused wire from the positive terminal of the battery and a wire from the negative terminal for your ground. 8 gauge or 4 gauge wire will be more than sufficient. An 80 amp fuse is recommended. The amp will also need a remote wire, which will be a 16 gauge wire that connects to a source that is 12 V when the key is in the "On" and "Acc" positions and 0 V when the car is off.
You'll also need a signal going into the amp. If you have an aftermarket head unit you will run RCA's from it to the amp. If you maintained the stock radio, then you can either get the interface that Pac Audio makes (C2A-CHY), or you can convert the signal at the rear speakers to a RCA level signal with a line out converter (LOC). If your amp has hi-level inputs, then you won't need the LOC.
Almost every car audio amplifier has a built on crossover. Set it for low pass (LP) between 60 and 100 Hz.
2. What's the best 10" sub for this type of application? Is one 10" sub enough to make a difference?
There are so many choices for 10" subs. The LX enclosures (Magnums, Chargers, and 300's) all have a max depth of 7". The Dayton H.O. subs that we carry work especially well in small sealed enclosures like the ones we offer for the LX's.
A single 10" off 400-600 watts will do much more than make a difference. It won't rattle the bum of the driver in the car down the block, but it will rattle yours. If you are after the boom of a pair of 12"s in a trunk engulfing enclosure off 1500+ watts, then forget it, you know what you need for that. However if you want bass that will make music sound like, well, music then a single 10" will do.
3. What's the best amplifier?
Again, there are so many choices. I'd recommend 400-600 watts per sub and sticking with quality names. The deals on the Hifonics amps over at woofersetc.com have been some of the best deals I've seen. The important thing to have at the forefront of your brain is matching the output of the amp to the impedance of your sub(s). If an amp makes 400 watts at 4 ohms and 800 watts at 2 ohms, then it could power a single Dayton HO. Or a pair of H.O.'s if they are wired in parallel. Check out www.bcae1.com for some more information on wiring possibilities.
4. Should I angle the subs so that they don't cancel each other out?
It's a common misconception that two speakers facing each other will cause cancellations. The short answer is that it is a non-issue and you could angle the subs any which way and not hear a difference. If interested, here is the long answer...What your ears hear is the vector sum of the two sources. That's why if two subs are out of phase with each other(one pushes when the other pulls), you will hear nothing, because they completely cancel out. That's an extreme case, but it applies for the subs being partially out of phase. Whenever there are two sound sources, there will always be some cancellation that depends on frequency. For a bass frequency to be completely out of phase, the difference in pathlengths(distance from sub to ear) needs to be 1/2 wavelength. It is common to have your subwoofer only play frequencies below 100 hz. The wavelength of 100hz is 22 ft, so it would have to be 11 feet farther away to completely cancel at 100 hz. At lower frequencies, the distance would have to be even greater. The pathlength difference with when the subwoofers are in the rear corners of the trunk or hatch is under a foot, so you can see that cancellation is not a major factor.
