I respond to myself:
It's a impedance problem. I use the Radial EXTC, to feed the Voodoo Lab GXC, and this crappy box have only 1K ohm input impedance at the loop return. ( The spec. says 10K but not my multimeter ) 100K – 1M ohm is what "normal" guitar equipment normally has, or?
The H9 working as an "expensive" buffer in this case and can feed the low impedance of the EXTC. My vintage efx-boxes can not 🙁
Putting a MXR buffer in front of the EXTC return input cures this…