HSU was also mentioned. and maybe the sealed SVS SB-3000 is a better comparison to the DD15.
If ported is tolerated, then the PB-3000 and HSU VTF-3 may fall into the ($1,000, used) category. HSU 15 looks seriously (way too) big, approaching the size of my 4.0 ft3 sealed 18" on the main system.