OK, I've had two cases now where a HomePod just would not soft reset. In both cases, it would refuse to connect to the WiFi network. In both cases, it kept complaining that it could not find the WiFi network. I then tried the soft reset. This means you unplug the device and then hold down the top of it for a looooong time until it goes from flashing white to red and then finally says wait for three beeps and then it resets.
In both cases, when I tried to set it up, it would fail to connect to the wireless network and dump me into the iPhone settings menu where the WiFi was working fine. I can't tell if this is some strange interaction with our UniFi network, but the only solution seems to be a hard reset back to the factory conditions.
You have to do this by plugging your HomePod into your Mac, then waiting until you get the Allow connection, and then going to Finder and looking for it in the Finder menu. (Strange I know!) and then you get the option to do a "Restore HomePod".
This actually seems to work, so you are warned 🙂
As an aside, the Stereo Pair thing works pretty well, when you bring up a second HomePod in the same "room", it will ask you if you want a stereo pair and which side is the new device. Works great.