Oh, I forgot to mention that the phisical termination provided in the box by Eventide, together with the T-connector, is actually 50 ohm…

Hmm.  We'll investigate why the internal termination isn't working.  In the meantime, do you have a 75 ohm terminator that you can use?

(As an aside, I looked at the H8000FW user's manual, and it says that the WC impedance should be 50 ohms; this probably explains why the H9000 manual says the same).