Out of curiosity...last night, based on others' comments, I took two of my four HomePods that I had located in different rooms, and created a stereo pair in my living room, as an experiment.
At the moment, they're not ideally placed in the room. But I will say the sonic difference between the mono HomePod I had in my living room and the stereo pair that I have now is astonishing.
Of course I was expecting some benefit/difference, but not that level - the difference knocked me out. I wasn't expecting that. There's no way I'm going back to a mono set up for that room.
Many thanks to the above members who advocated stereo pairing!