@cmach My two DD -12 Plus rest on birch plywood dolly's with extra-soft casters. The dolly's are topped with four inch layers of dense latex foam and approximately three inch layer of closed cell old Morey Boogie board foam.
I needed to rerun the Auto EQ and slight manual preset tweaks to maintain desired frequency parameters.
The decoupling greatly reduced the extra low frequency transmission throughout our dwelling while actually providing a more desirable presentation. I'd say elevating the units may have played a roll in the presentation itself.
Having owned a DD-18, I recall the manual didn't convey the importance of room positioning.
The manual made mention of walking around room with the mic and noting the greatest frequency spike while viewing the graphic display during the Sweep Tones, (page 23?).
With the DD Plus the subwoofer crawl is step one out of the box. Proper room positioning requires far less use of processing parameters and noticeably better synchronization with the room and the main speakers. Your DD is still a great unit.