I am in Ontario as well. My HVAC guy put one vent that I can manually close and open. My theater is in the basement and is 27x17x7.75
I have no return. He felt it would be fine and after 5 years it was. I only used the vent for cooling and have had very comfortable temps.
Only my projector is in the room as my equipment is in a separate room. With Ontario building codes / insulation / poly etc I have bee fine. I do have in floor heating so perhaps that would be the difference.
BTW. My HVAC guy was the best one in Windsor and no return in my room has worked out. My highest heating bill has been under $300 and my place is 4500 sq ft plus a finished basement. You may be good running no return. I have been