i think its because the pressure from being closed in and no flowing air causing the subs to produce less bass.if the box was ported it would be louder but compared to w the windows down it would still be less loud.the air flowing in and out of the cabin relieves pressure build up.
well if you take the back seat out there are options. i have a 98 gmc ext cab. and i shoved 2 re sx 18s in there with room for my amp and battery. runnin my jbl gto 1201.1/its a little underpowered but at 1 ohm its not too shabby.
i have this amp also. but i am tryin to sell it because it has a built in protection feature that really gets on my nerves. it will make your subs last forever but they wont reach their full potential.its a bit of a trade off. oh and i dont think you can run this one a t 1ohm. but you can run it...
just use the ipod to go through music.lol.i have the same HU and i never have problems.its much easier just to use your ipod to change songs instead of working out the controls on the unit