looking at an 18" Q in a 4cuft sealed box....would a us md-1d be too much for power, or should i look into the bl line?

It should be fine power wise with that as I believe it does about 1200W into 1 Ohm. With 4 cuft I would definitely have up to about 4lbs of polyfill on hand.... try the sub for a while as is and let it break in. Then after you are used to what it sounds like, try stuffing it with the loosely fluffed polyfil to bring the Qtc of the enclosure down and increase the bottom end performance some.

Thanks,

Scott

i dont have either sub or amp yet. still in the planning stages of things. its for the gf's car. she wants it to sound as good as my 15" ava but louder.... i have about 4.3 cuft before displacement to work with. this is going to be my fall/winter project as i have a seasonal job and need something to keep me occupied