I got the Raspberry Pi 3A+ last week and the HDMI to DVI adapter finally showed up on Friday. So I immediately hooked everything up and started to redo the setup (fortunately I'd tried to keep notes for most of the config changes). I had powered the Zero off the USB hub built into the monitor which meant I could turn both on and off with the monitor's power button. At first I did the same with the 3A+, but I got occasional "under voltage detected" errors. At first I ignored it as it didn't seem to have any impact, but then RetroArch mis-recognized the Xinmo controller and I knew a change was necessary. So I switch to using a standard wall wart and started over.
But I really wanted to be able to turn both the monitor & Pi on and off with one switch - ideally with the monitor's power button. If I couldn't use the built-in USB hub, maybe I could use the 12V jack used to power a speaker. In fact, before I bought the USB powered speakers I'd looked into some 12V audio boards for the RPi, hoping there would be a reasonably priced option (ha!) which would also power the Pi. What I wanted was something cheap which would convert 12V to USB power.
Then it hit me - cars are "12V" (not really), but I bet I could crack open a cheap (dollar store cheap) car lighter USB adapter, solder on a cable with the correct barrel connector and have a probable solution. A couple of hours later I had done just that - and it worked perfectly!
Moving up to the 3A+ has also increased the number of potentially playable games - although this time I'm starting out with just 44 from a combination of "best of" lists. So I need to go through those, make sure they work (near) perfectly, then make up the media for the front end. I also need to finalize the control panel artwork (I've decided to just use heavy-duty vinyl, so no need to try to cut & bend plastic) and then I can build the final cabinet!