Are all MIDI devices compatible with Quartet's USB MIDI port?

No. Only Core-MIDI USB devices will work with the Quartet's MIDI port. Most new USB music keyboards and drum pads are Core-MIDI devices, but this will vary from different manufacturers. Please refer to that device's Owner's Manual or specification to confirm class compliance with Core-MIDI. In general, MIDI devices that can be plugged into a USB port and used without needing to install additional software or drivers are likely Core-MIDI compliant.