Having heard both at length - the best difference I can describe is in depth and "punch". The SUB1 can easily extend as low or lower than the DD 18, but it's real strength is much tighter, punchier bass IMO. The Velodyne sounds powerful, deep - good rumble but the SUB1 is just the better sub for my money. Keep in mind 5 years is a very long time - the technology in the SUB1 is light years ahead of the DD 18.
Your room is large so 2 large subs would be a good idea.The sub 1 is much better than the dd18 ,at the prices quoted.For your room , 2 x sub 1 would be needed , or 2 seaton submersives.
Moving your main seat back 2 1/2 to 4 feet will improve sound also if possible .
I have run multiple subs for 15yrs plus and 4 for 12yrs and for most would recommend 2 max unless you have a big budget for HT.Better to improve front stage,amps better subs etc first.