ok....i'm a bit confused. so, your head unit's speaker outputs are running through a line level converter to the factory amp's "speaker" inputs? this really doesn't make much sense. but i could have it all wrong. if you're bypassing the amp, you shouldn't need line level converters at all! just run the pre-outs from the head unit to your amp in the trunk. right?
also, since this installer sounds like he was ON CRACK when he did your work....check to make sure they ran the power cables for the amp seperately from the signal line. you'll get a much cleaner signal that way. they should never run parrallel and directly next to to each other for long distances.
lastly, you shouldn't notice that much of a difference with the extra .2 cu ft for the enclosure. the bass will begin to roll-off a little sooner than in a smaller box, but your sub is propably happier with more air. beleive it or not, the air inside a sub box can get pretty hot as you play your system louder. the more air for the sub, the longer it'll take the air to heat up, and (potentially) the longer your sub's coil will stay cool enough to run properly (less distortion).
