Thanks for your input. I solved the issue, but don't understand "how" or "why" :-/
Here's my set-up :
my instrument (the Chapman Stick) has stereo outputs, going to a stereo preamp. Then :
– the preamp output A goes directly to the input 1 of the TF
– preamp output B goes through the Modfactor in mono (input 1). Then, the output of the MF goes to the input 2 of the TF.
Here's the trick : I thought I could have only one jack out from the TF to go to the amp : I thought the TF could sum both inputs and then outputs the mix of both inputs. In fact, no : I have to plug two jacks out from the TF and then go to the amp. Pretty bad news! When I plugged only one jack out from the TF (on the mono out) to the amp, here's what I described earlier : when the preset of the TF is active, it's OK. When I disengage the preset, nothing goes from the TF, except the signal coming from preamp output A.
Who have an explanation for a newbie like me? Is there an issue with my TF? Can this problem be solved in a future update?