Phone: Galaxy S7 Edge
Android version 8.0.0
Samsung version 9.0
Power Amp 3 build 301
I'm having the same issues with the battery drain. Can understand why it is difficult to fault find as it comes and goes. Phone running hot and a steep drop in battery are the trademarks when it happens though.
The battery screen shot is an hour of listening wired this morning. I keep my apps pretty clean so close them after use but maxmpz was still running in the background even after I'd closed Power Amp after using it. That is the second steep drop on the graph combined with a little bit of gaming (the gaming app dropped 4%).
The worst performance from the update to the app is wiping out 80% of my battery in 1.5 hours listening wired. That said I've also had the normal use from it (7 hours with a mix of BT and wired) and minimal battery drain.