A good friend had Duntech Princess (more similar to DAL SC-IV) in a 20x20 room with a 9' vaulted ceiling and they sounded fantastic.
HOWEVER, read any source on acoustic design and they all recommend against a square room (avoid standing wave reinforcement). I think my friend's room worked so well only because one end has a 6' wide opening into a smaller room plus an open door way leading to another room; in other words his room was not square acoustically.
I would suggest contacting Rives Audio or other acoustical designers to discuss your situation. And I will suggest that all Dunlavy designed speakers work best with placement well out from both the front and sidewalls. I would be more concerned with sidewall distance than distance between speakers (meaning 4-4.5' each side with 10-12' between speakers [centerline to centerline] in a 20' room). In my experience the 10-11 feet to the listening chair that Daverz suggested will be the minimal distance. Such tall speakers need distance for the drivers to converge, particularly time/phase coherent designs like the DAL.
HOWEVER, read any source on acoustic design and they all recommend against a square room (avoid standing wave reinforcement). I think my friend's room worked so well only because one end has a 6' wide opening into a smaller room plus an open door way leading to another room; in other words his room was not square acoustically.
I would suggest contacting Rives Audio or other acoustical designers to discuss your situation. And I will suggest that all Dunlavy designed speakers work best with placement well out from both the front and sidewalls. I would be more concerned with sidewall distance than distance between speakers (meaning 4-4.5' each side with 10-12' between speakers [centerline to centerline] in a 20' room). In my experience the 10-11 feet to the listening chair that Daverz suggested will be the minimal distance. Such tall speakers need distance for the drivers to converge, particularly time/phase coherent designs like the DAL.