you dont really need a certain amount. i like to keep it atleast 3 inches away from the flooor. unless your dealin with 8's or smaller woofers then you can move down. its a good concept when you dont ahve alot of cone area. and your woofer wont unload when u open the trunk lolwow nice man! realy love the window! little question about the down firing; is there a minimum amount of space needed to flow (or is that space also tuned to something?)
greetings from holland