seems by those measurements that you have a pretty decent area to work w/, a tad over 8.5 cubes from what i calculated
there are really way too many possiblities to begin to list what you could do w/ 400$, if i were you, 400 would get me a SE15 from RE @ about 200-220ish, then grab a jbl 600.1 or the like off of ebay for 180-200
but it really depends on what you want, dual 12's, dual 15's, single 10 ect ect ect, give us a nore specific idea of what you want