What are you looking to gain here...
Power to match the extremely efficient Klipsch......?
What is the size of your room...?
Mike's suggestion of getting a pair of Ultra 13's isn't bad, they might trump dual DD-15's in tuning flexiblilty, SPL, linearity, and possibly SQ.
On the other hand, with the DD series, you have a built in EQ, which adds to the ability to smooth out room responses. I find that to be a priceless piece of equipment. You could also get an outboard SMS-1 to do the same thing.
If you were to sell the DD-15, you could possibly buy dual SVS Ultra 13's and a SMS-1 for close to the same price of adding a second DD-15...
Hard decision...