9v 500ma sounds a bit risky given that four “mini-computers” are powered by it.
Pedal power supplies are fine with multiple analog-style guitar pedals (those usually draw a few tens of mA) but digital stuff draws more. Also those midi controllers have lots of LEDs judging from online pictures.
Often power supplies do not deliver what is written on the specs. It might refer to a maximum/peak draw or it simply is a cheap supply that can’t be trusted. It is usually recommended to have a power supply that can deliver at least twice of the current you expect.
Ksoloti/Axoloti Core drew up to 90mA when I measured, and if the MIDI controllers draw anything similar you’re already at 90*4 = 360 mA. Much more if lots of LEDs are on.
This is a rough estimate, mind you, and should be taken with a grain of salt.
Fun fact: there is a way to measure rough current consumption on Ksoloti Core: Measure power draw of Ksoloti