A bit more on the app and the Spark NEO device.
When you switch on the Spark NEO, it goes looking for the last Bluetooth device it connected to, and then other paired devices if it doesn't find it. This is regardless of whether the Spark app is running on the device. So if you want to change the device you want to run the app on, or which you want to play music on, you have to mess around with the Bluetooth settings connecting to the headphones. This is unlike the Boss Katana:GO which will happily play audio from any device that it is paired with. The main purpose for me is to play music on a device and play along to it, so this is, while not actually a deal killer, a significant inconvenience.