Play purchase (Premium) is per Google account. This means app can be used on multiple devices as long as devices have your Google account:
- please ensure proper Google account is registered on your phone and selected in Play app.
- use Poweramp Equalizer Settings / Restore Purchase / Google Play
Website purchase is per single device. You're free to upgrade / reset / reinstall app on this device, or upgrade the device itself. If app is used on the multiple devices at the same time, activation will stop working.