The Magus should supply up to 500mA with no problem. If the device draws more than this, there is a power switch which automatically switches off. You should then see an error message:
USBH PWR Fault.
Some of the devices we've tested with successfully are:
- Alesis Q49
- Arturia Keystep
- Korg NanoKontrol
- Akai LPK-25 Wireless (requires latest development branch to work)
I'll see if I can get hold of a Beatstep or OP-1 to try with. Linnstrument might be harder!