I read an interesting spec on the Delta Pro 15; qes-0.44, qms-4.73, qts-.40, FS-42 and enclosure of 2.25 to 3.75. Appears a very light cone and xmax 3.2mm. Efficiency is high as well. Perhaps a Carlson or a pair of Carlson designs? I read the Carlson design provides crisp & detailed bass better than reflex designs. (I know little about speaker box design as you can tell).