Stacking subs that go down in "quality", however unnoticable it may be, is stupid (to some extent).
Dont waste your money and buy a dd-15 or dd-12,
Rather, save it up and buy another dd18 to either stack or put on the opposite side of the room as your present dd18.
Why compromise for looks or potential salivation of the viewer? A pyramid style subwoofer is only a result of an obssessive disorder in the brain, known as ear-a-tosis. It doesnt make any sense, sonically or economically. Any audiophile, with even the smallest bit of class will tell you the same.
When you have the coin, buy another dd18.
By the way - I love the dd-18 SBW, Congratulations.