By all accounts the DD18s are pretty awesome subs. Reason for replacement? Do you need what little extra space smaller subs would provide? If it were me I'd just keep em.
For a smaller sub the SVS SB13 Ultra is probably the smallest with the most output. The next size up would be something like the Rythmik F15HP or PSA XS30. Not sure just how big your subs are and what you're trying to accomplish by down sizing.
Another option would be SVS PC13 Ultras. They take up only about 17^2" of floor space and are tune-able.
Klipsch RF-62II, RC-500, RS-400, SVS PC12+,
Def Tech SC8000
Harman Kardon AVR 1600
PS3, Apple TV, Sharp 70" Qattron