You could give your stuff to me until she turns 18!
But seriously I use baby gates. I've used them for the kids, grandkids and our pets. At one point we had gates everywhere. It doesn't look great but that shouldn't matter because your daughter is going to have that place a mess anyway. That's what 2 year olds do!
Maybe wall mount the bookshelf speakers if possible. For the sub, you might try changing the firing direction to the side that she won't have access to. I would be more concerned with your cabling. Make sure it is tidy and tucked as far out of reach as possible.
Grats on the new family! Family is where it's at!