On April 5th this year a retailer had the Apple HomePod on sale at $299, $170 off normal retail price so I bought one. I then connected it to my existing HomePod as a stereo pair & configured the Apple TV 4k to use them as the audio output.
This was mostly successful except often the thing would forget the HomePod pair & connect to the much less-capable TV speakers. Today, though, the Apple TV & the HomePods were updated to version 14.2 of their respective OS. A few updates ago, version 14 changed the HomePod OS from being iOS-based to tvOS-based since they're a more natural fit & integrate better in the Home app.
In the Home app I had two separate Rooms, one with the Apple TV, the other with the paired HomePods. In version 14.2, the only way to select the HomePods as the Apple TV's audio output is to have them in the same Room as the Apple TV. Then they can be selected as the default audio output, thus forming an Apple Home Theatre Surround Sound system. This means no more having to reselect the HomePod pair as the audio output at random.
But I found a glitch in this setup: the minimum volume setting was about 50%, not the expected 0%. Plus the highest volume setting wasn't as high as the speakers had managed before the update. I just couldn't figure it out, the Home app isn't really that intuitive to navigate. At one point I intended to remove a Room I'd created to test a failed setup & accidentally deleted the speaker/Apple TV Room instead. I the deleted the Room I'd meant to & the Home app then recreated a "Default Room" containing the Apple TV & HomePod pair.
I renamed the Default Room then tried again. Success! The volume control goes from zero (quiet) to eleven (too loud). This was the desired outcome even if the route was somewhat circuitous.