Values into system calls (RPi 4 Raspbian Buster Thonny)

[hitsware]

import os
import time
os.system ("sudo modprobe snd-virmidi")
os.system ("aconnect 20:00 128:00")
b0=str(hex(12*12))[2:4]
b1=str(hex(5*12))[2:4]
b2=str(hex(100+27))[2:4]
while True:
  print (b0)
  print (b1)
  print (b2)
  os.system ("amidi -p hw:1,0 -S 'b0 b1 b2'")
  time.sleep(1)

This prints out the right values (90 3c 7f)
But the midi doesn't respond
If I replace b0 b1 b2 (next to last line) with 90 3c 7f
the synth plays ........... ??????????