Socks may not be "the" answer but socks are definitely "a" answer. In other words, anything that restricts air flow through the port will change the ports tuning. There is no one answer but a whole range of them from no restriction to fully blocked. Since socks don't work you can jump all the way to something completely plugging like a wood plug with just enough foam or fabric around it to hold it in place.
Don't be surprised if even that's not enough. More likely you want to be looking at the room, speaker placement, and listening location.
Don't be surprised if even that's not enough. More likely you want to be looking at the room, speaker placement, and listening location.