Srdan01, My room is about the same volume as yours and a DD-15 does fine for me just filling in below 30Hz. However be cautious, as current Vel 2.2 software only allows crossover at 40Hz and above. Unless you can get Velodyne to FPT you the older 2.1 software, you will have excessive overlap between the sub's LPF and a full-range main speaker. Using the sub's EQ to fill in null points much above the crossover point is usually a bad idea.
Any Vel DD can be improved with a few internal mods to the plate amp & wiring.
Any Vel DD can be improved with a few internal mods to the plate amp & wiring.