I have a 10 inch sub in my trunk and it sounds great. Before I added the sub my system was missing something. Now it sounds good. Just enough bass to fillout the music.
It sounds loud enough to me without venting. Not obnoxiously loud that the car next to you can't have a conversation. Any reputable stereo shop should be able to put a sub in your trunk for you to try out. Test it out before you order the rear seat passthru.