In my travels, I have seen that a lot of Europeans and Asians live in very small spaces. Of course, the Japanese have their futon beds, which they can roll up and store in the closet. And the Europeans have "junior" sized furniture and appliances. Even in the US, many do not have the "luxury" of basement space. Space efficiency is required here.
But even living in larger places, I always spent 90%+ of my awake time in just a small area. Area that is just more comfortable and livable, we can call "good space." A large house also has a lot of "wasted space," the rooms and m2 that you don't use. Those extra bathrooms, extra bedrooms, extra basement space. That stupid entrance hall that impresses nobody...
Now I concentrate 90% of my efforts and investment on the "good space." And I just de-invest the "wasted space," minimal furniture, turn off the HVAC, minimal maintenance... This is space effectiveness.
So...as the listening room is the center of your family life, I think a paradigm change is warranted. You could allocate a given room's value to the amount of waking time you spend there. That means your "wasted space" basically worthless, and your small "good space" is a lot more expensive per m2 than you initially thought. Real estate brokers of course will vomit on this concept. However, as all the value of your house is in "good space" significant investment in making it more livable is warranted. And stop going to pottery barn to decorate that 4th bedroom nobody will ever use.
This could include more "radical" ideas than you are considering.
For example, the basement is wasted space, so my favorite idea is to install a few of those NYC-style equipment elevators. Those are the horizontal metal doors you walk over on the sidewalks in front of every little store in the city. Imagine pushing a button, the doors open and the the horns rise up from below. if you want to increase support, you can run downstairs and prop a 4x4 between the basement floor and the ceiling (actually the bottom of the elevator platform). If you want to save money, ditch the electric motors and install a manual crank system. If the basement is not tall enough, get someone to deepen it.
Another alternative you have mentioned is moving most of the electronics downstairs and monitor them via camera, thermal sensors etc.
Another alternative is just expand outwards for audio and baby toys.
I'm sure there are better ideas, but I hope this gets the ball rolling on some creative solutions to everyone's favorite listening room!
I might approach the problem a bit differently. Think "space efficiency" and "space effectiveness."