EDIT: after some troubleshooting, looks like I have a bad cable that doesn’t provide USB host power, so this should not be relevant to anyone else.
I’ve started testing some USB controllers with Magus USB host and so far it doesn’t look very promising. Magus itself is powered from 9V PSU (advertising up to 1.67A). All devices work if I route them through a PC instead of connecting directly to USB host.
Linnstrument 128 - doesn’t power. Manufacturer rated to use 0.3 A, but 0.5A is recommended.
OP-1 - it’s battery powered, but even with fully charged battery or USB charging disabled I still don’t get MIDI working. If I use USB splitter with external power (1A), it works. Disconnecting power or device and then connecting it again gives me the “USB Host unrecovered error” error message and MIDI doesn’t work until reboot. I suppose that’t the dreaded USB host issue that @mars mentioned before.
Novation Launchpad PRO (not the latest, but the one that had partial open source firmware support) - wouldn’t power from Magus (manufacturer recommends using 12V 1A PSU, but doesn’t tell how much exactly would be consumed on USB). If powered from external PSU it doesn’t see Magus. This is obvious because its default firmware switches to keyboard-like mode if it’s connected to anything, otherwise it plays rainbow animation.