so i was planning to put a DC audio XL 18 in my ext cab truck. but im tryna keep it as cheap as possible while still being a good system. i am now looking at a XL 15 instead. i know DC recommends 3.5 cu ft. now i want to have the box sitting in the back seat. it can go from the drivers side wall to just over the middle line and up almost the roof if needed. so how big is normally good? and what would be a good amp for it? on SHCA they just have 1 rms and watt rating for any size sub. what is a good tuning for it in a ext cab? i wanna get real low if i can but still have the regular good bass too. and lasty what power upgrades should be done? or is it better to just **** it up and do a 18?