i will try to answer your questions.
1. If your amp already has many build-in Caps than you don't need to add. Otherwise you might add Caps.
2. Window close the bass will travel through the tires and extend out to the road. so you can't feel the bass but your car in front of you or next to you for sure feel the bass. When the window open the bass will extent outside through window and that level is where you seat at so you feel the bass. Hope this help.