Personally, I would leave the wall open. Anything that creates parallel relecting surfaces is generally a bad thing acoustically as it supports standing waves. The more odd shaped the room, the better it is in preventing standing waves.
However, there is something you could do as an experiment at no real cost. Start out with the wall open. Don't trim the edges of the opening for now. See how your system sounds. Now, take a piece of drywall and stand it up to block the opening totally or partially. Just tack it into place. Now see how the room sounds. If you don't like it, you can finish the trim on the opening. If you do like it with the blockage, you can now finish the wall/door as you originally thought of doing.
By the way, this may or may not be an issue to you. But, many speaker wires cannot be buried in a wall or ceiling unless it's approved for such a use by government regulation. If you don't have approved wire, and you bury it, you will not pass an electrical code inspection. Also, if your insurance company finds out about it, they will not pay any insurance claim. It might also be an issue upon selling the house. I'm not preaching, but if you're going to do what you're going to do, at least be aware of the implications.
However, there is something you could do as an experiment at no real cost. Start out with the wall open. Don't trim the edges of the opening for now. See how your system sounds. Now, take a piece of drywall and stand it up to block the opening totally or partially. Just tack it into place. Now see how the room sounds. If you don't like it, you can finish the trim on the opening. If you do like it with the blockage, you can now finish the wall/door as you originally thought of doing.
By the way, this may or may not be an issue to you. But, many speaker wires cannot be buried in a wall or ceiling unless it's approved for such a use by government regulation. If you don't have approved wire, and you bury it, you will not pass an electrical code inspection. Also, if your insurance company finds out about it, they will not pay any insurance claim. It might also be an issue upon selling the house. I'm not preaching, but if you're going to do what you're going to do, at least be aware of the implications.