1. Behind the kick panel is best.
2. Same-gauge wire of branded vs generic, there's no obvious difference for the most part. You can probably only see the difference with the help of an oscilloscope, and only under certain conditions. Beyond that, it's largely based on personal preference.
3. It will depend on your current setup. Although, having the Big3 upgraded will also allow you to have a headroom for future sound system upgrades.