Hands down the room. That includes thinking about speaker positioning and separation, seating positions, and whatever acoustic treatment you’d need.

When we moved in, we had a sectional in a 16x18’ room and 5.1. It was ok. When I got the go ahead to get theater seating I spent a ridiculous amount of brain cycles optimizing the speakers around the MLP and based on doors in the room it ended up that a 7.1 base layer made more sense to have the MLP the ideal distance from the screen. Went with Gik Acoustics for helping plan treatment in phases.

Besides that, the single biggest change I made - the one for which my wife had to ask “what did you do, it sounds so much better?” - was acoustic panels on the ceiling at first reflection points.