Ok, I was on the right way with Live's Sends knob curves
The [pan2] object is made to drive a [*~] object not an exponetial knob
So I try to reverse the exponetial curve, creating a table with audio in and knob value, but I didn't manage to reverse it, my maths are too poor :-)
I the meantime I found the LiveCurve made by Stefan Tiedje, then I found the exponent attribute of [live.knob]
So I just use this instead of [pan2] to create a the curve I need.