You might think you are disagreeing with me, but you aren't. Hsu and SVS are just promoting the purchase of dual sub systems by saying four times the power is the same thing as four times the output. I guess you can mean that since output isn't strictly defined, but most people who are into audio tend to think of output as amplitude, not power intensity, so Hsu and Mullen's statements are a matter of a sales pitch really. If you press them on this point I think they will agree, perhaps reluctantly lol.
But I agree that dual subs are well worth getting, and in many circumstances two good subs can be better than one great sub, but poor me, I only have three pair instead of four like fancy-pants you. When I win the megamillions lotto tonight as I surely know I am, I am going to finally build my ULS-15 line arrays that I have always dreamed of, and then you will be sorry!