The DD-18 will go deeper and with greater ease. They are very musical and do not over power my room.
I previously used a single DD-15, but once I went to the stereo DD-18s, the bass no longer sounded forced. It is simply there, in an effortless and limitless way. It's the best way I can describe it.
Mind you, setup and placement is critical. Replace the standard feet with some decent brass spikes as well.
Good luck.