I have Alexia 2s in a NYC apartment which has your exact dimensions 16x21x9.
My speakers are 19" from the front wall at the rear inside corner of the toed in cabinet. The speakers fire down the long walls to my seat. The walls are dense plaster. I could move the speakers to within 18" for more bass reinforcement and that would work too, but once I move to 20", I lose too much bass reinforcement and gain nothing. That does not mean this positioning will work for you but it worked within an inch for my previous Sashas and WP 7s in my room. Good luck.